Hi, the problem is caused by the "Y Offset".
It works applying an offset to the base spline (similar to Editable Spline->Outline), but it fails because tries to solve intersections with the overlapped vertices.
Obviously those vertices are not overlapped because are at different Z, but this case is not taked in account because the algorightm works only in 2D.
One alternative, may be to enable "Geometry->Simple Y Offset mode". It uses the same method than Spline->Outline, which is more simple and don't check intersections.
Anyway, i just noticed it doesn't generate the same result than Outline. Probably the code has been refined in the latest Max versions, but we are using an outdated revision.
I've added this to our to-do list, to check it.
Meanwhile, i would suggest to modify the style to use two base splines (inner and outer), and prepare them manually using Outline.
Setting Y Offset to zero should solve the problem completely, regardless of the spline's complexity.