This is a bit detailed, so I'll give names to the worlds.

World = W1
World_nether = WN1

World2 = W2
World2_nether = WN2

I have synced the W2 and WN2 together, but when mobs/items are thrown through the portal in W2, they go to WN1. When things go through in WN2, they go to W1. Players are fine however.

Edit:
It seems there is another bug report that is a bit older still open. I'll leave this up to confirm that this bug still exists.

I've also had this issue but not just with the nether but also the end portal as well. Tested all through out 2018... poor test llamas didn't have a chance...
Confirmed that it still happens in 1.13.2 as well.

We have been having this same issue for a while now, do you have a link to the original post?

@ArchGoddess and @MrPistachios

It's #72 from 2013. To be completely honest, I doubt this will get fixed.

Duplicate issue of #72.