When I editing vehicle models in Blender, I always use "Mirror", "X Global" function to copy and reverse a parts to another side, such as rearlights or flashlights.
The shading is normal in Blender.
But the only problem is that the shading was also reversed in the game editor.
You can see the parts that mirrored has weird shading, it became dark when facing the sunlight, and became bright when it's in the shadow.
This problem has puzzled me for a long time, and I don't know how to fix it.
