Why didn't this Cho ult do true damage?

i have the 4 void buff. I've seen it happen a few times, usually its not on the first ult of the round like it was in this video. I'm wondering if somehow the buff gets lost when one of the void champs dies? Unless its supposed to work this way? which would be weird because no other buffs get lost when champs die. Maybe its due to having Yuumi on him?