Villagers are randomly despawning even when named with name tag.

The villagers have a clear path to their job site block, the bell and their home. Impossible for them to be killed as anything dangerous is walled off, (corners included as I'm aware of that bug as well) they have multiple iron golems and its on peaceful. 

There is no real process to duplicate just spawn some in and do your thing. I'm in the same chunk as I'm working around the villagers to improve the area so I'm not even going far enough away to warrant a despawn.