Villagers despawning

When I leave the chunk of the village (become unloaded) and I come back, sometimes i find that villagers disappeer. I thought zombies are killing them, but no proof since difficulty is normal and they hide in their houses during night.(and unloaded chunk)

It seems only these villagers disappeer: weapon blacksmith, farmer , clerik but not cartographer, librarian,that villager with arrows,armor and tool blacksmith...