Please help! Pics included

Took me a minute to see what you were referring to. That's odd. Have you tried to move the chandelier? It's almost as if that portion of the staircase isn't getting the lighting it needs. Sorry, it that's not much help to you. I've never run into this before, myself.

Raise the light source and see if that helps?

So, your game isn't properly reading the room correctly, rebuild the room with the Custom Room tool, it should fix it. LMK

Try moving the Chandelier as it might help with the radius of the lighting.