Mobs will instantly despawn if the player falls down an infinite hole.

In Minecraft, I want to build a space lair so I created a flat world with "the void" preset to make it like it is in space. I started building and spawned some mobs and I am in creative mode so they are not chasing me and I accidentally fell off into the infinite hole and died and it says that I fell out of the world, but here is the issue: As soon as I respawned, ALL of the mobs I spawned were gone! This does not apply to minecarts, boats or dropped items, but all monsters and sometimes animals are GONE!