Cured villagers despawn

I have a zombie spawner, which occasionally spawns zombie villager. I cure this villager, and transfer him to my iron golem farm. However, whenever I leave the Iron Golem farm area and come back a while later, the villager is gone. I'm sure that he didn't suffocate because of glitching through walls, because I had him centered in a room using water streams from all 4 corners of the room.

Perhaps the villager keeps some zombie properties - so that it despawns when the player is far from him.