In beta 1.16.0.51. any villagers could despawn. Even a villager with "Master level 5" profession, it also despawned. They're like 24 in my world and turned into 13. I just went far like five hundreds of blocks away and when I'm back, they're gone.
Same issue on:
-Samsung SM-T270R (Android 4.4.2)
-Samsung SHV-E470S (Android 5.0.1)
-Oppo CPH1803 (Android 8.1)
I agree with you this happened to me as well.
This is somehow related to world with custom biomes enabled. I enabled it without knowing what kind of feature was that back in 2020 (prob around nether update 1.16.100). Mine had one and now wipes off biome data every time I reload my world ever since I upgraded to version 1.21.40.x.
I am stuck playing with v1.21.30.x for stability.
Edit: It also appears to be an intentional world-breaking feature. I hope this will be reverted as it destroyed my world biomes.
[media]https://feedback.minecraft.net/hc/en-us/articles/30556182663437-Minecraft-Beta-Preview-1-21-40-23