I've been noticing that the villagers start to go missing after some time. 
The blocks they interact with are still locked with them, which makes it annoying when I am trying to build stuff up and having to place about 2 or 3 things before the villager I am working on will accept it. 
I am on peaceful mode, so it is not any mobs killing them. 
It's getting rather tedious working with villagers and I have seen that it's been an ongoing issue for a time now.