Variable Pipe creates non-manifold/twisted shapes

I have a bunch of arcs, and am using VPipe to generate pipes that are narrower at the ends and thicker in the middle. But randomly, some of the pipes come out twisted (see pic)

Any ideas how to stop the VPipe component from creating these twisted pipe shapes? As far as I can tell, all the curves have the same direction. I am relatively new to grasshopper, and am using a definition provided by Daniel González Abalde here