Mobs sometimes are not loaded by the client after joining the server.

Server is probably not sending them to the clients as all the players are not able to see the mobs.

The issue is still present after one player rejoins the server while others are playing, but when all the players in the area disconnect and reconnect after the last one has disconnected, the mobs will be loaded properly.

It happened to me twice: the first time my friend's wolf has magically disappeared and after some time it was found at the same place it was seen last time. The second time all the villagers in my breeder disappeared when my friend joined the server before me. Rejoining the server didn't help until we both disconnected and then reconnected.

The issue might be connected with MC-144603 MC-144564 MC-144558 MC-144609

Edit: Just a few minutes ago the wolf I mentioned duplicated - now there are two exact same wolves with the same name in the same place, both fully functional

Edit2: Just after joining the server I took a screenshot of how the cows in my farm disappeared from the specific chunk I spawned in. You can clearly see the corelation between other cows' positions and the chunk border. The screenshot is attached below.