It is not a plugin problem. As you know Maxwell is physically correct. The interpolation of normals (Phong) is an old-trick to get smooth surfaces with few polygons. This cannot stand with the Maxwell philosophy.
The worst situation occurs when the light direction is almost parallel to the faces. In this case there is a conflict between what Phong says and the real geometry.
While Maxwell tries to improve this situation as much as possible, the way to handle this issue is to increase the number of polygons.
This is one of the drawbacks if we want to keep Maxwell absolutely unbiased and physically correct. So please increase the number of polygons and you will get smooth surfaces. If you understand Maxwell, you cannot expect to get these things working as in the old-traditional render technology.
EDIT: please send the scene anyway to maxwelltech@nextlimit.com