Its even easier to see if you set “lower hemisphere is solid color” in the skylight and set that color to black, but i have no idea if thats causing it or just making it easier to see, it seems visible even with that off but its very hard to see.
Hey there @Quittzer! Welcome back to the community! Since you mentioned skylighting, I’m assuming this mesh is in a level and not the lighting test environment of the material window.
This looks to me like it is ambient lighting coming from the skylight itself coming from setting that lower hemisphere to black combining with standard ambient occlusion (or lumen occlusion) darkening things that would be shaded even with primary lighting off, though it shouldn’t apply to shapes like this.