Why this using sweep1?

The break happens no matter the diameter.

WHY.3dm (980.1 KB)

there is a slight kink (G0) transition there, is it needed? if you can make it G1 it will probably work correctly

I can see the kink when I zoom in close to the original curve I rotated. I’m not certain I’ve completely wrapped my head around the G labels, but I’m getting there. How/where do I find that info about an object, and then change it?

exploded the curves and use GCon

you could replace marked curves with tangent blendcurves, since these curves are G0 at both ends:

Thanks. BlendCrv and trimming with the created blend worked.