Swapping of Vein HW and FW segments after Addition of reference surface




I have some generated some veins using the interval selection and would like to control their geometry with a reference surface which is built from the vein segment midpoints.


The challenge I have is that once I add the reference surface under the "custom reference surface" section, the vein is swapping the Vein HW and FW segments. This becomes an issue because this swap is now conflicting with the manual polylines and points that I have already added to refine the vein HWs and FWs.


What could be causing this?