note: I apologize for my English, I'm from Brazil and I'm using google translator to report the bug

The problem occurs as soon as you load any world, all mobs that are in the initial carryover area of ​​the world disappear.

I realized this is happening when loading the world recently, I have a house on the edge of a village, which means almost the entire village is loaded when starting the world, all the mobs in that zone are gone: villagers, cows, sheep, dogs domesticated llamas, tethered llamas, iron golems, among others.

But, the most distant part of the village that does not carry when the world starts was not affected, some iron golems and three villagers were there, these mobs in particular did not disappear.

This issue also happens when a second local player enters the world.

Finally, this mob disappearing issue also occurs when returning from the Nether if there is no player previously loading the target portal.