I have made a building for cured villagers only and sometimes when i log back in, some of them just disappear, they cannot escape since i use iron doors to make sure they can't get out and the place is lit up so no mobs can spawn inside. This is really annoying since i spend a lot of time to capture and cure them. I play on a purely survival realm and i'm the owner.
This is the room from where my last villager disappeared.