I've been playing a survival map for one year. When the 1.16.5 update happened, I expected updates to take place on maps I hadn't explored yet.
Instead, what happened is that all of my villagers, animals (horses, llamas, cattle, sheep), maps that hung on walls (I had no copies made), and even the trails I made with my shovel (when you right-click on grass and it turns brown)—all of it disappeared. Completely gone. Vast roads I built are gone. Entire villages emptied. All horses missing. I have no maps anymore.
This appears to be a duplicate of MC-211308, and has happened because you have downgraded your world and doing this is not supported. Please see @unknown's, comment on the parent ticket for further details.