MC 1.16.5
iris-mc1.16.5-1.0.2 + sodium-fabric-mc1.16.5-0.2.0+IRIS2-rev.31828aa

Mod I found this with:
https://www.curseforge.com/minecraft/mc-mods/artifacts-fabric

Some of the Artifacts in this mod have very normal lighting effects with shaders off, but toggling them on causes some really strange results. I tested around 10 shaderpacks and a majority of them cause this to happen (though some of the low-performance ones like Enhanced Default don't have this issue). I did report this on the Artifacts page before posting here.

I've tested this with my mod loadout only being Fabric API, Iris/ Sodium, and Artifacts so its not from any other mods.

Looks like this:
https://i.imgur.com/gDoCpGQ.png