did u tried with bloom off already?

open console with tab and type in: bloomquality 0

I tried it, nothing changes. I detected that the problem affects walls also, depending on the sunlight vector direction.

The correct command for the console is: r.bloomquality 0

I've been looking out for that issue and it might be that I don't use bloom but I didn't notice anything really bad on the ceilings. Some of the floor pieces were reflecting badly back at mod release, but I changed those and everything should have flatter materials at this point. I will try to fully test this later tonight when I get on my test server.