It's a major step forward for humanity when they can finally live on a new planet. Maybe they went there out of desperation to survive, or maybe they simply want to explore. Either way, they suddenly lost all communication to people outside the planet and they will have to live without it. What possible reasons why they will actually forget that they did not originate on that planet and start living as if they have always been there?
This is normally done with the application of a few hundred years, technological collapse and a migration of the population away from any physical artifacts of interplanetary travel.
Technological collapse takes away all their records, even if they reinvent computers later they probably won't be able to read the old records. The technological collapse doesn't have to be traumatic, a population density too low to maintain the industrial base required would be enough to do the trick. They become subsistence farmers because there's no reason to be anything else.
That leaves you with a civilisation who have no concept of the ability to travel through space, a loss of history, and even if they know they came from a long way away, the lack of the concept of travelling through space means they just think they came from another country/continent.
Some book considered holy and written long ago claims some deity created that specific planet, and then all sorts of creatures and finally humans on it.
So the inhabitants of Eden think they have been expelled from the Garden of Earth, without ever realising that Earth is another planet.
It only takes a generation or two before that which may once have seemed foreign may then be seen as something that was always there. For example, in our daily lives, we don't think about how certain food items aren't native to our current homes, yet mere centuries ago, pineapples were a luxury.
We take things for granted, and that's something that can happen anywhere. In the USA, many people consider themselves as being American - they've lived there their entire lives, their parents lived there their entire lives, and the same for their grandparents. Perhaps a few more generations back their ancestors came from lands abroad, but that is all but forgotten. This can apply to a planet as well - once a generation is born never knowing those who came from across the stars, the memory will become a historical note. And if such history were to be lost, then those who have only known humans to live on their current planet will have very little reason to wonder about how they came to be.
A Fresh Start + Propaganda
The planet was settled by some (take your pick) Religious Fanatics/Idealistic Utopians/Persecuted Cult/Tyrannical Government/Other Group that wanted to cut all ties to old earth to Get Away From the Heretics/Create Plato's Republic/Escape Persecution/Form the Perfect World Order. The entire crew of the ship has agreed to keep the existence of Old Earth a secret, and assuming they keep to the plan, the only real issue will be references that weren't properly changed in the old books, whether to constellations that are different, or the other planets in the solar system, or to politics/religion/history that didn't get completely expunged.
Societal collapse triggers a drastic reduction is specialists which triggers a loss of knowledge. It has happened in the past, (the bronze age collapse), and a lot gets lost during collapses. There is evidence many bronze age societies lost writing, which is pretty big loss. Now oral tradition may preserve some knowledge they will quickly get lost in superstition and story drift, until coming from another world is just one of thousands of creation myths. heck just look at how many earth mythologies have humans come from another "world"
How to get such a collapse is myriad, from a persistent famine, to plague, to a magnetic anomaly happens and anything will electricity flowing through cooks itself.
There had been many attempts at colonization of other worlds. All of them failed. As it turned out, the settlers just couldn't cope with the traumatic stress of being separated from the home world by incomprehensible cosmic distances, and living under a strange, subtly wrong sun.
For this reason, a new colonization program was started, where the settlers were hypnotically conditioned (with the help of certain psychotropic drugs) to forget their home world and accept the new planet as their home. All data to the contrary was purged from all material taken to the colony, and after establishment of the settlement the ship, with all foreign resources, was shot back into space.
Interesting things will start to happen once the coloniats start to perform archeological digs, or attempt to trace the lineages of their planets species', and compare their genes.
Humans are seen as invaders who do not belong here. It is necessary to change this view to stress that we belong here, we have always been here, this is our home. Any suggestion otherwise gets dumps in the memory hole.
Otherwise your humans have no legitimacy and big problems with justifying why they occupy a chunk of the planet...
The knowledge has been systematically erased from every kind of database, over many generations, to create a kind of societal control. This has happened over so long a period of time and so thoroughly that even the people who did the erasing now believe their own lies. In fact that false history is now the only accepted history. There's not even a shred of anything else.
Look at real world situations. Religion, for example, teaches histories that its followers just accept without question. Dictatorships often re-write histories.
In order for entire histories to be re-written or erased it would have to be done on purpose, by the people in power, over a long period of time, for their gain. The scary part is that its not impossible.
they suddenly lost all communication to people outside the planet and they will have to live without it. What possible reasons why they will actually forget that they did not originate on that planet?
The loss of communication generated despondency, panic, despair. Some people realized that this "orphan complex" might stump the planet's progress to the point of threatening survival - people concentrating on reestablishing communications rather than trying to go on and "do without" - and the "Earth is Here" campaign was born. With time, some wacko came up with a new Myth of the Origins, and someone else decided to adopt it.
Many years later, many of these "adjustments" later...
A charismatic fanatic.
Give them a leader with very strong ideals, and instead of making him evil and destructive give him personality traits that would make it easy to follow. Strong, smart, fair, self-aware, humourus, and able to take and keep control over the majority of people. And his mind is set strong on that his way is the right way and the only way.
Playing into his hand is that people feel lost and insecure about their new home and they are desperate for someone to tell them what to do. Maybe many people suffer homesickness. Or some other reason that makes them feel bad. Let him lead "a new way" that discards old beliefs and knowledge, with the promise to feel good when they do it. This way does not necessarily need to go backwards, like the destruction of technology and living primitive, but instead a leap forwards, making current technology and knowledge irrelevant or seem naive.
What that could be in detail is up to you. In any way it leads to him deciding that old contacts are contra productive (like keeping the relationship with your ex alive; why are you hurting yourself? You know this is not leading to anything) and most people will join him willingly and throw out the past. But of course there are always people going against the mainstream (out of multiple motives) and you have to decide what to do about them. Do they have to die? Or can this small fraction form a new settlement and are allowed to keep the old ways alive? Are these two groups allowed to keep contact with each other?
If you want to discard of any contact to civilasation you can get rid of the drop-out in multiple ways. They could have a real accident, or an "accident" either arranged by the leader or by his followers of which he does not approve (but he only hears about it when it is to late). Or they can go back home.
And then you let three or more generations pass to make sure all "histories of before" are only fairytales.
If you want it to be more extreme, old histories will be forbidden and destroyed or alternated. But be aware that the reader will expect "some little bits and pieces" of previous knowledge to remain and to be rediscovered.
Some sort of chemical in the atmosphere that interferes with long-term memory?
This could either be a gas (say low levels of chloroform or similar) that interferes with memory formation, or a radiation effect - either permanent, or perhaps from a crash landing on the planet? - that actually directly modifies the body's epigenetic configuration and switches off the genes needed for long term memory formation.
Researchers are beginning to understand the chemistry of memory better: https://www.chemistryworld.com/features/the-molecules-that-make-memory/7765.article
Is time on the storyteller’s side?
My main thought is to compare the question to real history. Setting aside the question of human origins itself, the largest disconnect in human history would be the human migration to North and South America. It is generally agreed that the migration took place across the exposed sea floor from NE Asia to NW America. The timing is uncertain, but was somewhere between 13,000 and 40,000 years ago.
My point is, except for the Inuit people of the NW, no civilization in the Americas knew there was land to the west of the ocean. The memory had simply been lost. This is interesting. A look at Iceland as an example shows that we can be very good at retaining oral records for at least a 1000 years.
Another good research area that I know much less about would be the Polynesian peoples. I know most of them knew vaguely that they came from across the ocean, but I do not know what level of detail was retained, and how this varied by island.
The reason we need a form of technological collapse for an advanced civilization is because written records can last a very long time. As long as people can read the original language, the knowledge will simply be rediscovered. Once we have the population forget how to read, the knowledge can be lost permanently, even if all technologies are reinvented. For an example of this in history, see “Linear A” script. We know its alphabet, we can even guess at its pronunciation from Linear B (Greek). We have no idea what it says.
The colonists left their home for a reason. Their old home may have been perfectly good, but they disapproved of it. Perhaps their religion wasn't widely accepted, despite their insistence that it was the one true way to live. Perhaps they remembered a time when they had a just government, and now they've had decades of corrupt ones.
Whatever the reason, these settlers feel shame for their home. And they don't think they can fix it. They're moving to a new planet to start fresh, to build something that will be immune to the mistakes of their past. And they don't want to talk about it, because they were complicit with the old way of life. Instead of writing volumes making recorded speeches about how ashamed they were of their past lives, they're focused on the future. They left, there's no point in moping around in the past. Maybe they went as far as severing communications themselves, not considering the homeworld to be worth the breath to talk to.
Their children never really know what drove them to migrate to a new planet, so the knowledge is lost nearly immediately. They might have some small pieces of the story, which are told as ghost stories or passed on as little idioms that nobody really knows the origin of. But a true record of the past is gone immediately, and after the death of the original colonists the new society has no real way of finding out where they came from.
Let me turn the question around. A large fraction of humans (maybe even a vast majority) believe humans didn't originate on this planet, despite the fact that we have very solid evidence that we did indeed evolve on Earth. Some don't even reject evolution and still somehow believe they believe that.
Many creation myths involve humans coming from a different place. But the Bible is quite enough here. Humans were created by God in some place. They were placed into Paradise. Then original sin happened, and humans were thrown out from Paradise onto Earth. There was a time when some people believed Paradise was a place on Earth, but for most people, Paradise is "somewhere else".
Even outside of outright religion, you have hundreds of ideas about the origin of humans, or at least their influence by powerful non-human beings (or even the mere motion of planets in the sky against a backdrop of constellations!).
People believe all sorts of things. Even if your civilization retains the written word, stories get distorted, old lies and fairy tales become "hard truths" and new stories get created all the time. Obviously, it's even easier if there is no written word - e.g. the initial colony ship losing its entire library, people no longer being able to write without computers that no longer work etc. This has its place even in the hardest sci-fi - it's very easy for a civilization to lose all access to advanced technology, even without any explicit reason. It's hard to maintain a technological civilization, and people will quickly lose knowledge that isn't useful anymore. Stories about humans coming from some "Paradise Earth" will remain, but who actually believes in them will be more or less random - it will be just one story among many others.
This is especially true if the planet was seeded by Earthly organisms in the first place. Even as the civilization starts to recover and rediscover things like biology, they wouldn't really have the same questions to explain as we do. The big thing that creation myths set out to explain was where does all the massive variety of life come from, where do you get all those extremely complex ecosystems that hang in precarious balance, but (to your personal experience and the stories told) don't actually fail... It feels and looks vaguelly engineered, so you posit an engineer. It took a lot of thinking to find the completely natural mechanism that creates this diversity. But on a world seeded by Earth life, you would see it happening on human time-scales. Not only would there be little diversity to begin with, but the initial seed would quickly diversify to take advantage of various "pristine" environments on the planet. If the planet was originally lifeless, it would be much harder to figure out how vast geological time is. The most popular stories would probably be about organisms evolving to fit their environment, but also coming to the planet recently. It's easy enough to believe the planet is just 6000 years old if you ignore everything around you except for human stories - but in a world that was seeded with life just a few thousand years ago, this would actually be a good explanation (for a time).
Humans make up stories. They distort existing "true" stories. That's just what humans do, and you don't really need any justification for that. Creation myths would probably be even more dominant in such a world, because there's more evidence for them - a seeded planet would be a distinctly unnatural environment, and provide even more of an evidence that someone engineered it (which would actually be true in their case, of course). Plenty of people would claim someone created the organisms themselves too - and humans as well, since we're obviously so similar to humans. Some will claim all of this is foolish superstition.
"We're not from here" will be just one class of such stories, and completely natural to how humans tell stories. It's not a weird story - it's what humans have told themselves in most cultures on the planet over tens of thousands of years. The only way in which it would be special is that it's actually true for your people, not just an allegory or a fairy tale or whatever. But it would take serious effort to turn that from "a story like all the others" to "actually, that does actually explain things".
It should also be noted that until you have good communication across the whole planet, people will probably have myths about "not being from around here" even from something as "mundane" as moving from a continent to an archipelago. Losing communication is quite a big part of this.
Embrace that. People do think humans didn't come from their planet. How does that impede or help your story? How do your readers relate to that? You're showing what the characters think, or what they do. You're not explaining the world-building to your reader, he's supposed to figure it out on their own. There's people in a world. They have stories. Some of those might be true within the context of your story. Until you get to a point where it becomes obvious that you're talking about a "failed" colony of Earth, it's uncertain what you're really writing about - plenty of stories exist about humans of Earth, fallen from grace.
I think readers will believe this - the idea of a society that has forgotten its true origins is actually not uncommon. Two examples of similar forgettings are Heinlein's Orphans of the Sky and Hal Clement's The Nitrogen Fix. Both stories happen after a societal collapse and the loss of literacy.
However, I would actually argue that this kind of forgetting is not guaranteed, even given collapse and a long time. The natives of Easter Island for centuries have passed down stories of their seaborne arrival. But, to be fair, I'm not aware of what the stories were for other island peoples.
Pandemic Episode of Lost Syndrome
This small group of colonists chose a wonderful planet that was very similar to their home planet. The new planet, however, contained a very particular niche apple tree that could grow only in the one grove the colonists settled near. Because it is so geographically restricted, that tree had developed an extreme defense mechanism via a chemical that acts upon neurotransmitters to effectively temporarily interfere with memory processes. This prevents the local fauna that happen upon that grove and eat the apples from purposefully finding their way back later. This protects the trees from too much environmental competition from these particular fauna that routinely eat all the small saplings before they can establish into fruit bearing adult trees.
These apples look amazingly like the ones from home, and although warned against eating native produce until testing could be done, a young cook finds the grove and, in delight, gathers as many apples as she can and rushes to bake a special treat for the anniversary picnic celebrating their first year on the planet. Everyone loves the pies and all remark about what a wonderful reminder of home these apple pies are.
For some reason, the new small group of human colonists are affected only gradually over several months, and only in long term memory, which also ends up being irreversible. It affects only that first generation. They remember basic things about themselves like how to take care of themselves and each other, but forget quite quickly why they are there, where they came from and all their old societal patterns and rituals they had brought with them. The only ones who don't eat the apple pies are babies, so within a very short time, everyone thinks they've always been from there.
Eventually over time, someone finds the apple grove again, but the whole community is much bigger by then and only a couple people are ever stricken at a time so that the people soon connect long term memory loss with eating apples from that grove. It becomes a huge taboo to eat apples from that grove and they name the sickness the "lost syndrome" because those stricken slowly forget where they grew up and complain that they feel lost and homesick without knowing why.