Mobs despawn when sent to the nether

I play on simulation distance 10. In my drowned farm, the mobs are sent to the nether to clear them out of the mob cap more quickly. However, when I change from the overworld to the nether, even through the same portal, they are all gone. This also happens when I hit one of them to interact with them.

 

How to reproduce:

• set simulation distance to 10
• create a bunch of water streams that carry naturally spawned hostile mobs into nether portals
• go to the other side through the same portal

What should happen:

• mobs should all be in the nether

What really happens:

• everything has despawned

 

When I lower my simulation distance to 4 chunks, nothing despawns, everything gets transported to the nether, even when I go there through a different portal a few blocks away.