Possible kayn ult bug on blood moon

Ive been playing kayn a lot of blood moon map and ive been running into this bug often or i believe it is a bug. During fights ill damage an enemy champion and i wont be able to ult them, im not being cc'd and its kind of annoying lol cost us a game a few times. Also a few times some allies and enemy players and complained about not being able to ult on different heros like akali, idk if they ran out of energy or what they just said they couldn't ult either.