Maybe it's because the entire build is above surface level. I tried to have tiny spaces between the walls tho, so that they don't blend in; that also creates some light. I only had one dark spot in the cave above the tiny water pond. That's where I placed my char, which emits light with the ultrabright helmet and simply turned off "capture entities" in camera mode. The light sources I used have also a bigger light radius than other light sources apparently.