I've been breeding and trading with a lot of villagers. I have them contained in separate housing, mobs are unable to reach or see them. When I go away to mine, explore or even work on my projects next door, I come back and they've vanished completely. No traces of hostile mobs or even zombie villagers, just vanished. Their work stations are still in their respected homes, no damage to buildings or anything.  I haven't named them because I'm wasn't sure that would have been needed.