Hi Shayan- turn on points for the surfaces - you’ll see what is going on- this is a case where it would be better, probably, to have a degree 3 surface, 4 points, in the short direction. (Do your MatchSrf with ‘Preserve other end’ set to Position)
Or just manipulate the points on the degree 5 surface so that there is no reversal in the shape they make.
One thing I noticed is that your surface points and the points on the input curves are sort of 'all over the place" in Z. I would try to arrange points so that they are not ‘fighting’ the shape so much-
Red is my edit of your blue curve.
The curves have the same shape.
pgAudiSrfs.3dm (129.8 KB)