I am playing Minecraft Java 1.16.5 in a Realm. Only half of my villagers disappeared.
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I had 7 villagers in a (closed off, well lit) room with beds, 5 of them had jobs and had been traded with. I had 6 more villagers with jobs in a separate area who were all in 1x1 spaces on top of honey blocks, with job blocks in front of them but no beds nearby. I logged on after not playing for a day after a week of playing daily and the 7 villagers in a room had vanished. The other 6 were fine.
I'm not sure if the 7 disappeared because they were roaming free (instead of in a 1x1) or because their room had beds. The only other difference I can think of between the two groups was that the 7 were held in a room with iron doors in a sort of airlock while the 6 were in a house with wood doors.
No other mobs have vanished (we have horses, donkeys, wolves, etc.) just those 7 villagers.
I have the same issue. I've spent many hours in one of my survival worlds and out in a lot of hard work into building and other things. Then today when I tried to join the sleeping screen showed up instead and then the game crashed. I tried to make a copy of the world but the same thing happened. 😞
Still not fixed😞