[MCM-652] /spreadplayers permanently loads distant chunks

Partner Rank: 3

Verification builds: 1.11

Summary: When mobs are spread with /spreadplayers to distant chunks, those chunks are loaded permanently until visited. In the extreme, this makes the world entirely unplayable as many completely irrelevant chunks are loaded up.

Repro Steps:

Create a new superflat world in creative mode. Make sure coordinates are visible.
Grab a mooshroom spawn egg from the creative inventory and spawn 40 or so mooshrooms.

Run this command:

/spreadplayers ~ ~ 0 10000 @e[type=mooshroom]

Run this command:
```
/say @e[type=mooshroom]
```
Count the number of mooshrooms found. It's usually around 20. These mooshrooms are still loaded for some reason.
Close and reopen the world.
Run this command:
```
/tp @p @e[type=mooshroom,c=-1]
```
Take a look at your coordinates. You are thousands of blocks away. This mooshroom has no business being loaded, especially after a relog. You are also probably at Y=255, proof that ~~MCM-111~~ is not fixed.

Expected Results: After spreading entities, all chunks should be properly unloaded.

Observed Results: Some entities remain loaded permanently, even after a relog.

Screenshots/Videos attached: No