Grading infill TIN lines pass over feature lines

[Citadel2012]

Can anybody tell me why the triangles of this surface (composed of infills only) pass over certain feature lines and triangulate nicely to others? I set the tesselation spacing at 1' and the angle at 3 deg in the automatic grading surface. The TIN lines pass over my face of curb and triangulate directly to the back of curb, which is causing contour mis-shaping.

My workaround for this includes adding lots of elevation points to each line; however, the feature lines have no elevation points other than the P.I.'s and I would prefer to keep it that way.  I am really wondering why the triangles are behaving this way.

(win 7 64 bit, C3D 2012)

[Neilw_05]

I had this problem again recently as well in 2021. It was an infill between curved retaining walls that would fail. I never got it to work so I had to leave a void in the surface between the walls.