Hello there, recently, I've made an underground area for my villagers, and they seem to be despawning. I have 10 villagers, made about 10 workstations, 15 beds. 

Upon breeding the villagers, I have 4 new villagers. I went out, and when I came back, I was missing about 3 villagers. 

It happened again when I bred more villagers the next day, my mending librarian, and another librarian disappeared. The children now took the place of the ones that disappeared. 

The picture attached below is regarding the first incident, where there are now multiple empty beds. 

Environment: Stone room, well lit, peaceful mode. There are also iron golems inside. Villagers have no way of killing themselves. They cannot escape because the only way to go down is by a 2 by 2 hole. A ladder on one block, and water flowing into the room, so I can drive a boat into the hole and bring them there. 

They cannot suffocate. They cannot be killed by mobs, (peaceful). They cannot escape, mobs cannot fight against the water current, and also cannot climb ladders. 

Thank you.