Unsure if this is more of a problem with rethinking voxels or immersive portals.
using Immersive Portals 3.3.9 for 1.20.1 fabric

and rethinking voxels-0.1-beta5 (though through trying others it seems to apply to all)

when where is a portal connected to another dimension on screen there is no issues, however when there is a portal that points to the same dimension it is in on screen there is extreme flickering and weird black artifacts overlaid on the screen.
one weird thing is that if you are looking through one portal that connects to another dimension, and through that portal see a portal that connects back to the original dimension there is no issue.
its just when the portal points to the same dimension it is in.