The portal appears to unlink after loading new chunks in the nether.
This video was taken in a creative world with the same seed and portal location as the one where I originally noticed the issue.
Seed: -1542455230400304514
Nether Portal bottom corners at: -59, 79, 82 and -59, 79, 85
Overworld Portal bottom corners at: -599, 61, 762 and -599, 61, 759
I agree this happened because we deleted nether chunks, however I think it's still a bug. I believe it's reasonable to assume many people will be figuring out how to regenerate their nethers because of this update. I just think the nether portal generation should depend on whether or not a portal exists, not whether or not you have created one before. "Escape the Nether," as much fun as it is these days, is not something I would like to play unexpectedly.
I don't think this is a duplicate ticket that "Works As Intended." This error occurred in a new creative world in 21w03a that had never been updated from any other version. Furthermore, it only happens after loading new nether chunks (unlike the duplicate ticket). The portal links correctly if I don't load new chunks.