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 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.