I do believe that it is a driver problem and especially not a gamestudio bug, because as far as I know it just uses the directx api to load and compile .fx files and there isn´t much to mess up.

The best solution which you should use anyways is to not use a flat shademode, but use the default setting instead and then split your meshes polygons. As a result it will look the same on all hardware and better than it currently does, because it then uses real face normals instead of the currently used per vertex normals which don´t exactly fit the shaded faces.