Villager disappeared. Not for the first time

I have a trapped villager librarian on my friends Minecraft realm that despawned. This happened previously in a villager breeder that I built and about 5 of them just despawned and never came back. This is extremely annoying since the librarian in question is a cured zombie that is selling mending as well.

What I expected to happen was...:
My villager(s) to stay trapped and never despawn (there is no way for him to get out or get attacked)

What actually happened was...:

They have despawned

Steps to Reproduce:
unsure how to recreate as he was trapped there for a long time