Hi Sven, when you trim a surface including with an isocurve, it modifies the trim boundaries but the underlying surface remains the same.

There is some description of this here:
https://moi3d.com/faq#Q:_Why_does_show_points_work_for_some_objects_but_not_others.3F

If you want the surface control points to end right on your isocurve trim, you can use the ShrinkTrimmedSrf command to shrink down the underlying surface so that any surface that is outside of the active trim boundary will be discarded.

- Michael