Alot of the problems explained here have to do with the imported geometry and it face normals. More precisely, the direction the quad, or triangle was created in. Sound strange? Think about draing a square. You caan start on any corner, and make a square. But on a quad, it does depend on which side the square is started. Can't explain it, don't know how to fix it, just know that it's true. Amapi does this too. Not so much with the shadows, but with the direction of the quad, causing the mesh to interact differently. I would recommend trying to run the object through "detriangulate" (I think that is what it's called) andd see if this corrects the problem. Worked for me before. It's available in free stuff, under utilities, or at least it used to be.