Things in RTC that do not work as expected (compared to Dungeon Master) or at all! Please read previous posts carefully to see if the bug has already been reported.
A small alt wallset in a larger area will use the wrong shading colour to shade distant objects in the other wallset.
I'm standing on a single alt wallset tile. The chest in the distance uses the shade of that single alt wallset (black). It should be using the shade of the original level wallset (light green):