Steps to reproduce

1. Collect a large number of non-persistent mobs within 24 blocks of the player. For example, run a spawner-based mob farm without killing the mobs.
2. Save and quit.
3. Make sure the world is set to simulation distance 6 or higher.
4. Reopen the world.

Expected result

The mobs would still be there for you.

Actual result

The mobs are gone.

Analysis

This is just like MCPE-66818. It is due to the fact that instant despawn chunks are defined as those chunks which are bordered by any chunks that were not ticked on the last tick. When you load a world, none of the chunks around you have been ticked yet, so on the first tick every chunk is an instant despawn chunk.

1. I left my character within spawning proximity of a Skeleton Spawner for ~2.5 hours.
2. This generated hundreds of Skeletons.
3. I wanted my friend to join and watch as I killed all of the Skeletons at once, using a Splash Potion of Healing.
4. At some point while "AFKing" my Microsoft account became disconnected, this made it so my friend was unable to join me with the world in it's current state.
5. I pressed the "+" start button.
6. I properly saved the world and let the saving process complete in entirety.
7. I pressed the Nintendo Switch's "Home" button and went to the Switch's "Main Menu".
8. I highlighted the Minecraft App.
9. I manually closed the Minecraft App.
10. I ensured my internet connection was functioning properly.
11. I relaunched the Minecraft App (I do use a physical cartridge but did not reset it in the cartridge slot and think it's irrelevant to this software glitch).
12. I allowed my Microsoft account time to "log in".
13. I reloaded the SAME Minecraft world I had just left less than 1 minute before, with HUNDREDS of trapped Skeletons.
14. Upon regaining control of my character, I found that my Skeleton farm's trap "kill-pad" was completely empty...All the entities had been erased at some point while logging out, or logging back in. This NEVER occurred before this new 1.16 Mob De-Spawning algorithm change.

I suspect that this has something to do with how players are inserted from, and removed from the world while loading/leaving worlds. I believe that the world "loaded" while my player entity was not "in" it, yet...this allowed the mobs to despawn as no player was "within" their radius to keep them in the overworld. I also think that mobs spawned from Dungeon Spawners, or Spawners in general...should be exempt from the "Natural" mob spawn algorithm or have different values of some sort...They are not "Natural" mob spawns; they're forced due to the mob spawner and player proximity, not darkness or biome conditions.