The black connections points at the end of the heating cable and between the red heating cable and the black cold lead are engineered connection points and WILL heat up and it is crucial that BOTH of these connection points get fully embedded in thinset and/or mortar.
Since these connection points are thicker than the heating element, best advice is to chisel out a small groove for the connection point to sit in, in order to lower it's profile.
DO NOT DAMAGE OR REMOVE EITHER OF THESE CONNECTION POINTS. THEY CANNOT BE REPAIRED.