Villagers in a artificial village despawned. Going to the nether never caused villagers to despawn, and going to the end once to kill the dragon didn't either. However on another trip to the end where I was there for around 3 hours getting elytras and shulker shells likely did. I noticed around 6 villagers despawned when I returned and put all my loot from the end away. They were unfortunately villagers I wanted to keep. All of the villagers that did despawn were all traded with at least once so their trades would be locked. One of the villagers that despawned was also name tagged. I've seen other things on YouTube related to villagers despawned after the release of 1.16 and supposedly there is a loading glitch when returning from another dimension where the game doesn't know where to place mobs that were moving.