BabaJ

It's twisting because your points twist.
Each face(polygon) are made of the 4 points - so if you turn 2 of those points it would naturally twist.
If you want a turn in direction you could add corner square polygon that would allow you to continue the change of direction.

If I missed something in what you are trying to do, just to let you know, two of your images don't have the proper links and are not showing; you could also post a hip file which makes it easier for many to see what you're trying to do and offer different solutions.

Maybe provide a hip...something is 'odd' about that.

I routinely use the Join SOP and haven't as of yet run into the issue you have.

It always 'works' for my situations.

You could create an attribute based on the points UV on its' prim with the vex function xyzdist()....and use that attribute in a sort SOP.