In multiplayer mode, sending an entity through a nether portal makes nearby monsters despawn in the overworld, if there is another player at a distant place in the overworld.

How to reproduce:

1. Play in multiplayer mode
2. Build a new nether portal (for example, at (10000, 100, 0) in the overworld)
3. Place another player at a distant place (for example, (10000, 100, 10000))
4. Summon monsters near the portal (at the overworld), surrounding them by fences
5. Go into the portal
6. Go back to the overworld and confirm the monsters are still there
7. Go to the nether again
8. Throw an item into the portal
9. Again go back to the overworld, and confirm the monsters have disappeared.
10. (Try again 4-9 without any other players in the overworld)
11. (Confirm the monsters are still there after throwing items)

I understand that this is caused by chunk loading triggered by the thrown item, but the chunk loading should be work somewhat like spectator mode, which does not make monsters despawn.