Is it a mod compatibility issue?

No

Minecraft version

1.20.1

Describe the issue

When portals are placed in a certain way a weird black line appears between them.
How to replicate:

Make a structure "1" (orange) appear inside of a structure "2" blue

1. Create the first portal in-front of the structure "1" like this:
   
2. Create the second portal at one of the sides
   
3. Observe the black line
   
   
   (The black line doesn't appear when these portals don't "overlap" from the viewer's perspective)
   

The glitch appears with fabric-loader-0.14.21-1.20.1, fabric-api-0.83.1+1.20.1, immersive-portals-3.1.0, Minecraft 1.20.1. Also appears with sodium installed.

Probably fixed in the next version.