Is it a mod compatibility issue?

No

If it's a mod compatibility issue, please provide the link(s) to the incompatible mod(s)

No response

Minecraft version

1.20.1

Describe the issue

Immersive Portals causes some interesting portal frame generation, this one is one of the best.

It happens also in vanilla world generation. Maybe amplified by my combination of Terralith and Tectonic.
Maybe use vanilla portal frame generation and then place portal there?

Vanilla always generate 2x3 nether portal which ignores shape difference so ImmPtl cannot use vanilla portal generation.

The screenshot seem to be non-vanilla dimension. Please provide an example of abnormal nether portal generation in vanilla dimensions.

Some parameters has changed to make this less likely to occur. Probably fixed.