Hi Andreas, sorry for the late reply. Next time you encounter that problem, please reverse the faces or Normal of the material in your modeling software, then sync back to D5 Render.
For example, in SketchUp, if I import a model with the wrong side, then in D5 the grass will grow on the underside after I turn on grass material or scatter plants. After I reverse the face in SU then sync to D5, the grass will become normal. This should work for other modeling software as well.
And I have this result. The grass is applied on one mirrored object only and not on the underside for the other mirrored object. I guess this is a separate topic haha.