zed got nerfed????

So this is what piss me off. I am playing a rank game using zed then something really "funny" happens. I used my ult death mark on akali and that means she should got a mark on her head. After my ult and leaving a mark on her head she w and when she appear after the w again which is not more than 1 sec my death mark got disappear. That makes me dead because my ult suddenly got vanish. After ward I went to 1 on 1 with yi ( which is still that match) I used my ult on yi. Yi used his q after i ulted and the death mark got vanish again while my R is on the cool down again !! WTF ???? This bug bring me back straight to bronze 5. Please Explain riot

(THEY GOT NO and )