This is for a medium/high fantasy magic setting with a medieval flavor (a D&D campaign setting) and am trying to get a grasp on expected long term effects/reaction by repeated worldwide devastation that happens on a cycle and a mechanism that would allow nearly in its entirety that it was just a legend or never really happened. Or that it is known by some people ( sages, researchers, secret organization) but nothing is ever done to prepare.
The idea is that dragons return to the world briefly for a few months or years and spend the entire time destroying anything and everything they can. These dragons are exceptionally powerful.
For the first cycle the world is not prepared and already in a form of dark ages dealing with other problems. Dragons were thought to be extinct and have little in their way then mysteriously vanish and the world is left on its own to recover naturally. New cities, new kingdoms, nature reclaims the waste.
Then roughly 1000 years later (timescale flexible) for a second cycle the world is taken by surprise and it happens again. The first 2 occurrences can be explained well enough but after that (by 3 or 4) someone is bound to see a connection.
Like a perpetual cycle of apocalypse through post-post-apocalypse. I hate to tie a parallel here but similar to the Matrix without the concept of "The One".
Main Question: When knowledge of the events would be recorded or passed down in a legitimate way that would be recoverable and actionable by a world population, by what mechanism might I be able to avoid or ignore the usefulness of this information?
I would prefer to avoid civilizations developing dragon fighting weapons and dragon dooms day bunkers or perhaps they wanted to but were somehow prevented from being able to, every time.