I might be wrong here, as I do not use my Hunter very often, but I have definitely hit a backstab and have it not do extra damage or kill the opponent, but I feel like I have also caught the melee recharging faster than it should. I propose that perhaps the two nodes are crossing frequencies so to speak. I think that in some instances the Backstab is acting as Fast Twitch instead of it correct perk. Again, I might be wrong about the Fast Twitch thing, but Backstab definitely messes up sometimes.