Inside an old world of mine (in which I haven't activated the experimental features or anything like that), suddently all of my mobs and entities disappeared in my base, this includes passive mobs, horses with their leads, villagers, minecarts and armor stands. All of the mentioned were secured not to be on a world border and this happened all of the sudden after loading the world far from my base, returning and looking that they disappeared.

It didn't affected all of the mobs since bees and a cat a little far away survived, and before loading the world, a day before the bug, I crossed a nether portal to travel far away inside the nether.

I would upload a screenshot but I don't think it's very useful to see a screenshot of lonely fences that used to have animals or the 1*2 hole where I used to store my villagers, but now empty. If needed I could post the screenshots