Isotrim removing Faces outside of Curve Boundary

I created a Nurbs Curve using a grid of points made in Rhino, so that the Surface can be flexed using Sliders connected to these points:

then i want to create Sub Surfaces using Isotrim so i can select which Faces to Bend independently (as well as flexing the entire surface):

so that you get this:

however, using Isotrim creates a larger field of faces like this:

so I have used SubList and List Item to select the Faces that are inside of the Nurbs Curve:

and sometimes when I move the original points in the Nurbscurve using the Sliders, the Isotrim Sub Surfaces are not responding correctly:

does anyone have a solution to this problem? it seems like the Isotrim field of Faces gets sometimes (incorrectly) redefined? Is there a way to permanently delete or omit the faces from the Isotrim that fall outside of the Nurbs Curve?