Mobs keeping disappearing of my worlds

Hello! So this has happened to me before, but all my mobs in my worlds have been vanishing. Recently, in my main world, all my pets, villagers, and farm animals vanished, even if they had been nametagged. I had most of them for over a year, so I was pretty upset over the loss sentimentally. When I tried to get some pets and villagers again, they were gone when I logged back in an hour later.

So now I’m on a new world set to experimental (the prior world was not) and was on it for 3 days, and recently attained several cows, 2 horses, and several bees. I then entered the nether and returned 5 minutes later to find them all gone.

This is very inconvenient (and slightly emotionally draining, since I name and get attached to ever creature I meet in this game 😅) and I would like it to be fixed before I attempt to make any more progress. Please help! And I’m sorry I can’t send a visual, but it’s kind of hard to take a picture of nothing.