If you push a hostile entity that doesn't have a name or item currently on them through a nether portal they will instantly despawn.

Expected:
All entities that get pushed through a nether portal will travel to the other side and remain there.

What happened:
They despawn.

To reproduce:
Create a portal in the Overworld and go through
Push entities through the portal from nether -> Overworld.
Go through and notice they're all gone.

Repeat with entities that have been named via name tag
they go through perfectly fine.

If the chunk is loaded, as if another person is in the Overworld next to the portal where the entities are expected to come through, all the entities will travel through to the other side perfectly fine.
They only despawn if the chunks are naturally unloaded.

Video example of whats going on:
https://www.youtube.com/watch?v=kKirUyKV8ks