Mirror modifier - the mirrored half is darker I am working on a complex project with different 3D models I downloaded from the internet.

I have to mirror many of them relative to the global x axis (I used a plane as mirror object), but as you can see the mirrored half is darker both in solid and rendered view. The material is the same, the lighting is uniform, the normals seem to be correct and there aren't double vertices.



Also, I would very prefer to not apply the modifier 



Here there is a simple .blend with one of the objects that give me this problem:
I found the solution: the issue was caused by the plane's rotation.

It seems like the Mirror Modifier doesn't work when the mirror object is rotated, so I needed to apply the rotation of the plane to make it work