My launcher closes all the time, unexpectedly, without saving the world. I don't know why this happens, but I assume it's because I get too far away from other players that are in my world. And I believe this is the reason that my villagers have been disappearing, though i'm not entirely sure.

I had four librarians in a room, with beds sat next to each other and lecterns two blocks away from their bed. The roof is three blocks above their beds, and there are only two entrances/exits. The first has two trapdoors, so that i can get in and out without the villagers being able to exit. The second is just a single trapdoor on the top block, and is always closed. 

When this had first happened (when one of them disappeared) I was travelling to the stronghold via elytra and rockets. I got too far away from someone on my world and my game closed. I thought nothing of it so I continued to the stronghold, and when I had gotten back one of my villagers were gone. I looked everywhere and he was just gone. I later then found that a baby villager had appeared in their room (I didn't give them any beforehand). Then a few game days later after I had filled the missing villager's spot with the baby, my game shut off again. I got back on and went to check on them, and all of them had disappeared except for one. No random baby villagers this time, just gone.