Overexposed highlights and incorrect motion blur with mental ray for Maya motion blur may be caused by the framebuffer Data Type. In this case, change the framebuffer Data Type to a float framebuffer. For more information, see Framebuffer.
When rendering with motion blur, sometimes parts of deforming geometry will fail to blur. This is because sometimes, during animation, Maya creates incompatible changes to vital mesh data (number and order of vertices, normals, uv indices, polygons). You need to ensure a consistent tessellation for mental ray.
Turn on the "keep tesselation" flag on the smooth node, and do not triangulate.