My villager disappeared

Put the summary of the bug you're having here

What I expected to happen was...:
I built an iron golem farm, with 3 vilalgers in enclosed glass cases, beds, room above, everything needed for nothing to go wrong.

What actually happened was...:
After a day one of the villagers vanished, villagers should never despawn, and it was in a locked glass case, so it couldn't have gone anywhere or have anything happen to him.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...