When a villager sleeps adjacent to another, they push each other and start to float away, going through solid blocks. Can also happen if a villager is sleeping and a baby villager jumps onto the bed the villager is sleeping on. Lost one of my nitwit villagers this way. The villager may teleport back when he wakes up or the bed is broken if he is still in render distance. I have also noticed that sometimes villagers lie on the floor when they first go to bed, then float onto the bed. Seems to only happen to adult villagers.