I had a small room dug out for my villagers (about 20 villagers), it was a long rectangular room filled with torches, lights, and various work stations. There was no way for any villager, adult or child, to exit, nor was there a way for any monster to enter. I also had 2 farmer villagers in a separate room dedicated to farming. I have been working on a project across the hall from where my villager rooms were and for months there was never a problem, whenever I went to check all of my villagers were there and so were the farmers. About a week or two ago, after having finished a part of my project across from my villagers, I went to check on them and there was only one villager left, all of the others had vanished. I checked on the farmers and they were gone as well. The 'village cats' who had been in each room were gone as well. A few days later I checked again on my one villager and he was gone. Nothing in their room changed, their beds were all there, their stations were there, there were no lighting changes and no way for them to have escaped they just vanished. It is very annoying because trapping zombie villagers took a very long time and converting them took even longer.