I was having a similar issue with a mesh in the 3rd person shooter template level. After modifying its settings in the mesh editor, it became black in the main viewport. The scene is all dynamic lighting, and I was trying to set up a Light Propagation Volume, so not sure if I was doing it incorrectly. Then I enabled “Dynamic Inset Shadow”, and it was lit. That was associated with the scene having dynamic lighting, I think. So, perhaps it’s a setting(s) associated with the lighting setup, and not UVs. Have you tried using Visualization Show flags, or debug overlays? They’re all in the Viewport buttons at the top.