I guess the SubD doesn’t “care” where your UV Seam is, so much like any geometry, it might get pushed around, if nothing is constraining it or holding it, which in this case, could be a support loop.(hope i got that right)
Adding the support loop DOES sharpen the edge though, is there no way to tell the SubD Modifier “hey here´s a UV Seam, don`t move that”?
