Villager turned into skelleton

Brought two villagers from a village to my base, trapped them in simple containers.

Couple of minutes later i see an Zombie arm, through one corners of the container, opens the containers and find a normal Zombie, and a skelleton instead of my two librarians.

 

Started reporting this bug, and when i went to screenshot the containers with Zombie and Skelleton, they were both gone!  Hahaha

They probably despawned since they were turned into normal mobs, and were not named.

Bug is: how did the Librarians turn into a normal Zombie and Skelleton.

Edit: There were no torches or lightsource inside the chambers, but can zombies and skelletons spawn inside a 1x1 area that allready have a villager inside it?