Villagers despawn or disappear.  I will have a village of 20 or more. I will leave and when I return over half will be gone. Remaining villagers instantly begin mating and soon there will be over 20 again. If I leave again they too will disappear.

I have the village walled with no possibility of wondering off. The villages are well lit and leveled off so there are no canyons or lava. There are no portals either. I have tried on peaceful mode the same thing happens so it is not pillagers or zombies. I have plenty of beds and workstations. This has happened on multiple worlds. No point to try to level up a villagers as they disappear before they become max level or shortly afterwards.

Newly spawned children become unemployed as adults as they do not seem to be taking the previous villagers jobs as though they are still there though they are not.

Animals in fences that are 20x20 or less also disappear as such I cannot breed sheep or chickens. Once again no sign or wolves or ocelot. And this happens to cows and horses too.

Nothing disappears unless I leave then when I return many of the mobs have disappeared.