When creating a grading group you can set the tesselation distance. Then, when you create a grading object, it automatically tesselates the feature line it's based on. You get similar functionality when you add a feature line to a surface: You can provide supplementing factors to tesselate the feature line.
But, when you add a feature line within a grading infill, it doesn't tesselate. The work-around is to add the feature line to the infill surface using supplementing factors. But I suggest that when a feature line is created within an infill it should tesselate according to the tesselation settings of the grading group.
