Novel black mesh problem

I’ve made four identical interconnecting rooms with decorative wall mouldings. A rectangle of moulding above the door renders correctly on one side of the wall, and black when on the other side (see attached). The problem is the same in all four rooms. When I copy the wall and trim, and paste it into a new level, the problem disappears.

Lightmap UV channel is at 1 and lightmap resolution is 128 for all the moulding meshes. All have the same material applied and all receive much the same light.

Why are these four rectangles alone rendering black? I’m stumped!

Any ideas gratefully received. Thanks!

Have you double checked the unwrapping is okay?

Stece_T Very many thanks for the prompt feedback.

The meshes are fine, the UVs are well laid out and channel 1 is enabled.

I’ve now fixed the problem by replacing the “bad” rectangles with copies of the “good” ones. Why this should work I have no idea! Both good and bad are the same mesh, with the same settings applied. They’re identical. This is what I just don’t get.