so far the villagers have made it much further along than any of the others they are nametaged and master where as before i had nametaged novice for the breeder and those despawn after about 20 mins. The current villagers have remained well over 12 hours with constant activity in the area.
1.15.2 I have this issue as well. i have a bee hive and few other of the ones you build. I now have no bees at all. if i want them again i have to use spawn egg to replace. This has happened 3 times. The bees just get further and further away.
I have upgraded to 1.15.2 and without any leveling the the villagers are still despawning. I did notice this wasn't as likely to happen if the villagers were master. So I upgraded them all 3 to master and this is the longest I have went without any issue. Setting afk till morning will comment back when/if they despawn.
in 1.15.1 I can also confirm that villagers are despawn without any trace at all even when maxed out and name tagged. I have a similar effect when i am killed by an entity and I go back to that area the mob have completely disappeared. I use the /back command to go right back to where I died and if in the overworld with no roof the mob will have despawned. I have yet to do any testing on 1.15.2
Seems like the issue is fixed by making the villager master and adding name tag I currently have all the villagers that i did like this.