Overt menstruation is actually not that commonly found, it's pretty much only found in primates (and a few others). Most animals have covert menstruation, where the lining is reabsorbed into the body instead of being shed.
how does that help, since we are talking about human-people who should have overt menstruation? Well, since you're tinkering with people anyway...
At some point your people had medical science to both identify and remove at least most of the reasons why overt menstruation was beneficial to human-folk, and the capability to modify human menstruation to covert - probably to minimize discomfort and/or messiness, though I make no claim this would actually work (might not've worked in the story, either, depending on how you want it to go). It doesn't even need to be everyone, even having a few people so modified might work out with some time for forgetting.
If this happened before your people mostly switched to artificial reproduction, and if this did solve at least some of the most obvious symptoms of menstruation, then whenever your people went around suppressing periods (and/or fertility) some or all of those with this modification might have gone unnoticed, since they were already not-showing the signs of fertility the usual ways. Well, they might even have had some signs that were overlooked in the absence of more obvious ones, like a period. Once people were using artificial reproduction regularly, any covert fertility would simply have gone unnoticed, it might even be a known "medical issue" (hormonal spikes, cyclical bodily issues) that just is not known to be related to fertility, as most of those so inflicted don't reproduce naturally anyway.
So when people were moving from mostly using artificial reproduction to removing the option of natural reproduction, some of those already modified might have not been sterilized, or the sterilization method might have not worked quite correctly due to the difference in the biological processes caused by the switch from overt to covert menstruation. Or even sometimes the sterility malfunctions, but is corrected at puberty - since bleeding will panic people who don't know why. So only those who have the sterility mechanism malfunction and also don't show signs like bleeding for some reason would slip by those watching.
Actually, it doesn't even have to be a deliberate modification, if some mutation or medical condition causes covert fertility, that is suppresses external periods, then all it would take is for someone with that condition to also have a malfunctioning sterility whatever.
It might take some careful world-building to have all that running in the background to convince readers without either info-dumping or leaving it handwavingly unexplained, but how that goes is up to you. Maybe a few mentions of people with hormonal conditions, that only later gets confirmed as the same thing your person has? underground teeny-tiny resistance to the sterilizers who can explain, eventually? Occasional vanishing of people who did have both, but got caught reproducing elsehow (while your person manages to skip detection)? Later testing (once your person is found out) finds a handful of others who had the same thing but by chance managed not to get pregnant?