all versions up to 6.6
If an object in a scene contains only one face it is exported to MSTS by far too dark. It can be workarounded by splitting the face into two faces but it is abit odd. In my case this affects LODs for far distances for tracks. (the base can be reduced to one rectangular face, but always needs the trick of doing 2 triangles, btw the rails-object are two railtops, in other words 2 faces and work fine)

Yes, I have also found that I need to Triangulate certain faces to get the lighting to work the way I expect. I guess I just got used to it as a solution and I no longer consider it a problem.

Pete, first of all you are right, it is definitly a minor glitch.
I'm on the way to battle against polycount and found this. Amusing is that if the single face is a triangle, you need to split it in two triangles. In other words it does not depend on the form, but on the number of faces. And I personally mentioned this to give Richard the chance to evolve from good to great to outstanding software :-)