Villagers despawns whenever the chunk gets unloaded.

"How it works"

1. When the player unloads the chunk where villagers are in.
2. Some villagers escaped out of a room which had no way to exit. Meanwhile Some villagers disappeard even they have a name tag. 

"How to find the bug"

1. Get 20-30 Villagers in a room .
2. Go 100-200 blocks away ( mainly depending on simulation distence ) from the room . Or Go to the nether or end.
3. Come back in the room again. 

While it seems to be a small bug but it affects the game so much.