Span direction in dynamo slabs is floating

I have been placing slabs from lists of 3D points along the boundary using dynamo. First I place the string of lines for the edge from x,y coodinates and then elevate them to their 3D position.

This has been working nicely but I have noticed that the span direction line isn’t part of the resulting slab and ends up in a sort of floating position if you edit the slab. I had ignored this to date because it didn’t seem to cause any problems (if it isn’t broke don’t fix it mentality) but I found one slab that was throwing an error when I tried to edit it and discovered that the error disappeared if I deleted the floating span line.

Is this working as designed, or could I be doing something wrong?

you can refer to this:
https://forum.dynamobim.com/t/span-direction-translation-and-rotation/54165

seem you can only set the rotation