[Gameplay] Zed ult does less than base damage

Wingsofdeath was streaming a zed game and, after buying Blade of the Ruined King, his ult never did over a hundred damage, even after displaying the "guaranteed kill" mark on targets and dealing well over half the targets hp in damage. He believes it might be linked to the BotRK active. This occurred twice in a row: first against Ashe, then soon after against a Kindred (no ult).