I noticed something strange about the ambient light for a long time. Now, I think the direction is wrong, bottom to top instead of top to bottom. So, I make an experiment: A close box in creative at 12h (tod 12), with different textures both on floor and on ceiling without any parasite light to have a good base for a comparison. You can see the result on this screenshot :
We can notice that the ceiling is always lighter than the floor, regardless of the texture used, which means that the light comes from the floor towards the ceiling.
I miss something?