the reason: a line has infinite normals, same as curves, but curves usually have curvature and as such a principal direction. Plane calculation is unpredictable.
the solution: If you want the plane normal to the surface for a specific point on a curve, you just need
- point at the curves parameter (t)
- tangent at the curves parameter (t)
- the closest surface/brep normal from the point
If you create the crossproduct of both vectors(surface normal and curve tangent) you will receive a 3rd vector. Creating a new plane -> origin=point, xAxis = tangent and yAxis = crossvector yields the tangent plane. Try for yourself.
Hope I could help.