Villagers in minecarts despawning

I have a secured, well lit building with several villager librarians. Each one is in a minecart next to their lectern so they aren't able to move.  Twice now the entire group of villagers has despawned.  No hostile mobs have entered the building and the villagers have not gotten loose and moved anywhere.  I have traded with each of the villagers, but they did not have nametags.