Pat

It should be noting that similar behaviors to this bug also happen with water in cauldrons, so perhaps this is working as intended. Nonetheless, there is definitely the possibility that this is an oversight or otherwise a bug.

...That would make sense. I never put together that it suddenly worked after I went through myself, and that was why my cat was suddenly able to go through, but now it all makes sense.

I would like to remove this issue, it seems to have been resolved

Yes, I myself am able to go through no problem.

I went back to check, and in 1.16.2, this bug does not exist.

The same happened when the mobs were not placed directly into the portal; as opposed to having their walking interrupted and then teleporting through the portal, they just walk right through. I tried it with foxes, creepers, villagers, sheep, pigs, and zombified piglins.

During the creative mode test, I spawned them directly into the portal, while when I was in hardcore, I pushed villagers into it myself. I can test in creative if the mobs being spawned outside the portal, then pushed in would change the results when I can play again tomorrow just to make sure. Either way, I know items are not working in any normal situation.

Found something similar on my map.

In my newest hardcore world, I had the goal of getting every advancement, but then this bug hit me after I killed the dragon. Thanks for reporting it!

I might want to add that when I used the same seed in creative and killed the dragon, the gateways did not generate that time either, unlike with Bruce earlier in this thread. Screenshot attached.