Villagers teleporting

I built an iron farm, with 10 villagers suspended in a glass box 2x4x3 dimensions. It works, but often a villager goes inside out when I get out of the world or when I get away from the place. This makes iron farming impossible hence everytime I have to put the villager inside again. Last time it completely disappeared: I guess it fell down and died.
So villagers are passing through glass when I spawn there or something similare, since I find them out when i get there.
Obviously there are no holes, so it's a bug ...