I was sending villagers through a nether portal for farming purposes but when I went through the portal I noticed that neither the villagers nor the minecarts used to transport them were there (in the overworld). I tested some more using items like dirt and they seemed to go through from overworld to nether but not the other way around. I reloaded my world and everything was sitting where it should have been but for some reason it wasn't before reloading. My other portals were working fine and as intended but for some reason this one wasn't. I even tried moving it and destroying the portal then relighting it but it still didn't fix the issue. I now have to reload my world every time I send a mob or item through.

I'm not sure how to reproduce this as it is a very hard thing to test.