In my testing I've only had it happen with the melon side texture and the Vanilla Normals Renewed resource pack, where it is sometimes using the _n & _s textures for magma. Magma still uses the correct textures.
As far as I can tell, it seems to work correctly about 20% of the time, though it could be affecting some other texture.

Tested with:

• OptiFabric 1.8.16
• OptiFine 1.16.4 HD U G5
• Vanilla Normals Renewed V0.9.9.9 & V1.0.0, both are affected
• Various shaderpacks, in case it only happens when shaderpacks configure OptiFine in specific ways. Affects all that I've tested.

I'm not sure if other resource packs are affected. I have not noticed it in any other resource packs I've tested so far, but it could just be affecting some other texture.

For testing I've made a simple shaderpack to display the _n & _s textures: showTextures.zip
Screenshot of issue:

Screenshot of expected result:

Do you get this with OptiFine being used standalone too or is this just an OptiFabric thing?

I was unable to reproduce this issue in standalone OptiFine when I created this issue, but oddly enough when re-testing it today it seems to have started happeing with standalone OptiFine too.
It also seems to be happening around half the time today, both in standalone OptiFine & with OptiFabric.

Given the random nature of the issue I'm guessing I was just getting lucky when initially testing standalone OptiFine, and unlucky while testing with OptiFabric.