Thanks @torreyjproctor for the file.
I must admit, that I don’t know (at least not yet), what causes this, but if you follow these 4 steps (again, I don’t know why this works), it should look good:
Change your Autosmooth Angle from 30° to 15°.
Add Custom Split Normals Data.
Clear Custom Split Normals Data.
Change your Autosmooth Angle back to 30°.
I tested this several times and in different Blender Versions, so it should work. Please let us know if that solves it.