Issue only occurs with shaders off - does not occur with Complementary Shaders turned on

Seems that you are confusing between sodium and iris. The shader is provided by Iris. Does this issue occur with sodium but without iris?

Now it seems I can't recreate it even with sodium+iris. I'll test if another mod is causing the compatibility issue

Further testing seems to indicate the issue only occurs with Sodium+Iris+Lithium

Never mind, Lithium isn't the issue - Iris is the cause
The issue manifests differently every time I restart Minecraft: sometimes the player model is half-culled vertically, sometimes horizontally, and sometimes the player is completely invisible in mirrors (and sometimes the issue doesn't occur at all)