Villagers disappearing for unknown reasons.

I was in the Nether, Once I went back doing my thing, I notice that one of my villagers disappear. This is the second villager disappear. The villagers were enclosed in a 1x1, with their design table in front of them while so their bed in the back. The height of the 1x1 hole is 2 high.  My nether portal is out of the chunk of the villager trading hall.  I don't know the reason why they disappear though.