Hi Danny, right now the surface combining only happens for a basic plane which you'll get from an extruded line. Even though the arc shape makes a planar surface when it is extruded since it does not have just the simple 4 corner point type of surface structure it does not get converted into a plane primitive and that's the ones that get combined currently.

I'd like to do some tune ups in the future to make surface combining happen for more cases but the particular case you have there might be somewhat difficult because the surfaces you want to merge there don't share the same UV layouts. So it means the simplified combined plane has a different UV parameter space layout in that area after it's combined, I don't remember if the combining mechanism is going to handle that type of thing or if it expects the combined surface to be more compatible in UV structure to the uncombined versions.

- Michael