I don't know why, but suspect it has to do with the symmetry. Can you split it into two splines?
I can but its going to give me an issue on the back and then I have to figure out how to achieve curvature with the spline in the back. Could it be that the surface it aligns to is flat?
Delete the contraint and instead use the "Tangent Face" constraint instead. Also try to "relax curve" in the Property manager. Also another tip is to only create one 3D sketch spline per sketch.