Both side’s material is M_Rock_Marble_Polished from the Starter Content. Both brushes have been aligned, as you can see by the pattern. The left side is lit by a stationary point light, the right side is lit by a stationary spotlight, pointing directly down. The passageway has been made with a subtractive brush.
Do you have a lightmass importance volume? what density is your lightmap? what lighting setting are you using (preview, medium, etc…)? do you have reflection captures there at all?
This could be many things.
If you’re building with Lightmass, then no there isn’t - you’ll have to combine the meshes. Lightmass distributes the lighting build accross multiple threads which unfortunately means you sometimes get different per-mesh results.
I can’t find the thread I want… but there is one around somewhere discussing it.