Question: 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.

Answer: 
Maintenance costs.

All of those dragon fighting weapons cost money. All of those dragon doomsday bunkers need upkeep. Even if they just sit around, someone has to watch them sit around and keep them useable. Or replace them if they become unusable.

It's easy to justify those costs for the first few decades after the dragon attack, because people will still remember it first hand. Then it'll get slightly harder, as people who remember it first hand start dying. Eventually, people will only read about it in books, at which point it will seem like a distant threat that is unlikely to occur again. 

When that starts happening, some enterprising young prince or duke will say "Yeah, we don't need to be spending all of this gold on dragon survival bunkers. Let's take that money and spend it on Necromancy For All instead!" And so, 1000 years later, there will be necromancers giving out free healing spells on every street corner and no way to repel the next dragon invasion.
A classic case of "We have the knowledge but not the will to use it." The political expedience where the cure is more disagreeable then the disease. How badly do you want the bearer of bad news to suffer in order to save the population?
One likely scenario is that a minor nobleman/knight's kingdom/village/area of influence was devastated in the distant past and a blood pact/family curse compels the descendants to look for signs and portents of a "reawakening" of the dragons. Now that the crisis is eminent, the "hero" has to inform/warn the public. Of course the family name is in disgrace so nobody will listen to their warnings of coming danger. This is made worse by the fact the protagonist is a drunk or hears voices or did some sort of faux pas so is totally discredited by the powers that be. One thing he can't be is a beggar on the streets as he would zero standing in the community bases on class/social standing. Another typical scenario is that a young monk is tasked with spreading the warning the masters have discovered/saved of the impending disaster but is socially awkward or that sect is currently in disfavor by the religious hierarchy so he falls in with a group of travelling actors/performers and disseminates his message in the form of a morality play to the masses after escaping from some harrowing adventures. The last scenario is where the powers that be are in denial and attempt to kill the messenger because it would disrupt or inconvenience the rulers and their decadent lifestyle.