Panels appearing lighter

Hi, does anyone have any idea why these panels might be appearing lighter, regardless of the lighting conditions and after building Production quality lighting?

In the attached screengrab you can see a series of lighter boxes in the middle of the darker boxes. All of them are simple static meshes. They have the same material, and their lightmap res is the same. The darker bit aren’t shadows. Why do they look lighter?

It’s because they’re separate objects. Is there any reason they need to be separate objects rather than one object?

I needed separate objects for setting up various interactive BP elements elsewhere in the scene. Can I combine them now? Or is there a setting I can change to unify them a bit?