Even though I selected the affected version to be 1.21.4, I started experiencing this bug/feature in 1.21 in July through a wither skeleton farm I was building. I had built a nether portal for wither skeletons to go through and didn't link it up with any portal in the overworld yet. I built the killing chamber on top of the nether roof and went through a portal that I had made in the killing chamber. In the overworld side, I encountered 100 wither skeletons that I had to kill, while wondering how they got there. I finished killing them all, and saw that there was a nether portal that I believe was created by wither skeletons.

STEPS TO REPRODUCE

1. Create a creative world with cheats on and build a nether portal of any frame size, from your standard 2x3 portal to a 19x19 portal frame size, and light it. DO NOTGO THROUGH IT, as it needs to remain "unlinked"
2. Spawn in any mob that will path find to you in survival mod, such as a husk.
3. Set the gamemode to survival, and stand where the husk is able to go through the portal.
4. If done correctly, the mob should go through the portal, even though the portal was unlinked in the first place.
5. go through the portal to make sure the mob is in the nether.

Video of me reproducing this issue:

https://youtu.be/nUCwiiMEAac

it should work in a standard unmodded vanilla world as well.