half kill on siegel

hi

today i made a duel with a tank and doing stab on him (blood stab) i made some lethal effect (half kill) but it didnt show on him or on me did they change the way of making half kill with this skill ( i mean like old backstab) or didt i missed an effect preventing them to lose directly hp or even if they dont have cp to be half kill too ? i dont really know if its a bug so sorry if i say usless thing