I think you're guilty of an incorrect characterization of those results as an "issue".
Well yes, although I did expect them to be different, not just for one surface to have close to a zero brightness value while the other is close to one.
for a light direction nearly parallel with one of the two surfaces
I must admit to not giving any thought.
Also as you say, the surfaces are at right angles so I guess it does make sense that the difference in light influence should be big now that I think about it.
In any case, I think I'm starting to come to grips with this now so I wouldn't say this discussion has been completely in vain (for my sake), albeit based on ignorance. So thanks again for all received input!