I had a zombie villager wich i turned back to a normal one, gave him a job(Librarian), levelled him up and named him "David"(Just to ensure he didnt despawn). I kept him in a very secured room, very lighted, with one more villager and he just despawned. Sometimes it would happen when I dismounted my horse and he would just vanish. Sometimes restarting the game would load the mob again but sometimes this wouldn't do.