Yes, we have seen this problem because other users reported it. But unfortunately, we don't know the cause... it seems some bug of 3D Studio when the geometry of Forest changes to face the camera. But as we have not access to the motion blur source code (it's part of 3DS), we cannot debug it.
I will do some tests again, and will try to find some new details about the issue. We also will contact with Autodesk to report the problem.