[GAMEPLAY] Morde Ult Cancelled by Illaoi Ult

At 18:43 in this replay, i ult illaoi with morde ult, at the same time illaoi turns her ult on, and it says "cannot be disabled." My morde ult went on cooldown, while not actually activating or doing anything, and illaoi's ult worked as usual. Is this a bug or is this how the interaction is supposed to be?