Mariux are you sure you
never see such an issue? I certainly have it and have seen evidence in numerous screenshots from others. The lighting issue affects corners and penta/tetra surfaces, and can be dependent on viewing direction. For example I have some corridors with the interior edges slanted/smoothed using a row of pentas and tetras. Two of these surfaces will exhibit the issue, but the surfaces on the opposite sides are correctly lit.
I have noticed that some sides of some block shapes map the texture "backwards" or inside-out to the surface; this is probably only noticeable with some texture packs or certain textures. I suspect that the lighting characteristic is related to this, such that the "incorrect" visible lighting is actually the way that the opposite side of the face (facing the interior of the block) would be lit. It's a bit difficult to explain without illustration; will require some experimentation with special textures to clearly illustrate and confirm.