I split a surface and delete the cut parts, and then when I tried to RebuildUV, or plug it as geometry on Grasshopper, the output still covers the cut parts of the base surface. I tried to see if there are hidden objects, but there are none. How do I remove those remnants?
By default, splitting or trimming a surface will not change the underlying surface. If you load your geometry in Grasshopper and use the tools that operate on surfaces, this means it will use the untrimmed counterpart of your trimmed (or split) surface.
Just note that it’s not always possible to shrink a surface to the trimmed edges. In those cases, an “invisible” part will still remain. The underlaying structure of a NURBS surface always has 4 edges…
-wim