Every mob in render distance despawns sometimes when loading a world

When I load my world, every once in a while when I get into the world almost all of the mobs will be gone, and (I think) only mobs within my render distance got removed. Even the nametagged mobs disappeared.

So far this has happened twice, I didn't report it the first time because I thought it might have been a fluke, but then it happened again. If you're wondering, the dates were 12/3 and 12/24. So far, this has only happened to one of my worlds.

Some things that might have affected this: 1) My world is pretty large filewise and 2) These both happened after making a copy of the original world, but this didn't happen every time i made a copy. Also, both the original and the copy are affected once it happens.

I don't know if this is a bug or not but I put it here anyways cause it's really annoying