I’m not a programmer and I don’t know how these calculations work behind the scenes in Rhino, but in my opinion, the weight value of 1 of all control points should be kept unchanged after changing the degree. As noted before, even the slightest change of CP weight disables the “Interior shapes” option and leads to an unwanted dense output blend surface.
If you watch my last video above, you will notice that moving one of the end control points up (making the input edge curved) actually fixes the bug. Then, once I moved the same control point back to its original position (the input edge became straight again), the blend surface was still clean, even though it was overly-dense just a moment earlier using the same straight edge as an input.
For some reason “Loft” does not suffer from the same bug.