Items under a 3D floor take their lighting information from the main sector and not the light value under the 3D floor.
I had wondered why the items in my well lit 3D house were a bit duller than normal - almost as if they were outside in the dark, and why my stuff in a dark tunnel appeared to be glowing slightly.

Player HUD sprites are correctly affected, but the player sprite (just like all other sprites) is not.
Example WAD posted to show the effect.