This was an interesting low-level display bug. I was working up a study for bends on a press-brake for this bent. Perhaps I will make a video tutorial on the workflow. Anyway…
From the right, the forth object was extruded and Boolean split using a small plane where I want my bend to end. I flowed them along a curve. The result was a polysurface as it should be, but depicted as a extrusion.
I boleaned them together in the 5th object, and it’s still a polysuface in disguise, with no shown naked edges.
The last object was the same object exploded, and made again into a solid, displayed as I thought it would be.
[Also interesting: Rhino adds a few more control points in the result than the original spline has, though my tolerances are quite high, and the result, as checked with zebra and enviroment-map, looks very good. I would guess that internally, Rhino converts the spline which is a joined segments of a 1 and 2 degree curve into a 3 (or more) degree curve. Yet, if it didn’t, the result would be more like in the first object. The local option is interesting, but not helpful. In this case it would be helpful to pick the segments of the spline to allow/disallow bending, or just not convert the curve before the operation. Like I stated, this is minor stuff.]