There are occasions when a blend surface operation places the handles where they’re not useful without a lot of tweeking. What causes this, as shown?
And even after repositioning the handles to what seems correct, I still get a small problem:
My guess is the problem is caused by one of the surfaces having a zero length side, which causes some ambiguity at the corner.
Zero length because of overlapping control points? Trying to match the surface I get a direction indication that’s off.
All surfaces in Rhino have 4 sides. It is possible for one or two sides to have zero length with all control points along a zero length side coinciding. This may cause problems with the math used in various operations.