I bred two villagers to create a baby villager, waiting for this villager to grow up. I then locked this villager in a room with a zombie. This turned the villager into a Zombie Villager. I then gave the Zombie Villager a Name Tag in order to stop it from de-spawning. Once I had done this I turned the Zombie Villager back into a Villager and gave it a profession as a Librarian. Shortly afterwards I left the Villager and was mining roughly 4 chunks away. When I returned, the villager had vanished, despite it still being locked in the room.