add breakilnes to terrain model

I am trying to add breaklines to a terrain model, I go to Terrain<Feature Management<Add Features, first I click on the terrain model than I click on the features to add, it appears to add them but the triangle edges don't change to meet the new breaklines I've drawn in.

When I go to try to add them again it reads "Element is already is this terrain model. It cant be added" See attached screenshot.

How come the triangles did not change to meet the new breaklines?