I have a completely self-contained village with walls, ceilings, no way in or out save for iron doors, and lit throughout to prevent any mob spawning. I logged in last night on my switch to find that half of my villagers were gone. A few weeks ago, I had one villager inexplicably disappear, but yesterday, five of them vanished. Buddies on my Realm confirmed they were all there on Saturday, so it happened sometime Saturday night or Sunday.