When looking through a Nether portal using Immersive Portals, a demon's skin renders normally. It also persists when both clients are in the same dimension, but ends after relogging either client and after a short period of time.

This issue can also be found here on the Immersive Portals mod's repository.

Does this also appear in Singleplayer? I tested it today in Singleplayer with the most recent versions and it seems to work correctly