Isotrim completing partial sphere

I have made a solid union between a few sphere, and then isolated one of the faces that I wanted to subdivide. The problem I’m having is that when I isolate a face of the new brep, the isotrim brings back the entire original sphere. Any help appreciated!

Isotrim (7.0 KB)

Isotrim works on surfaces. Every trimmed surface is not really trimmed, there is just trim data which hides part of the surface from you. So commands and components in Rhino and Grasshopper which work on surface data (iso’s, paramter space, ect.) always acts on the underlying surface data without trims (that is how nurbs works in general, and is also why you can untrim).

Thanks for the explanation!