Leblanc ult mana cost bug? [Gameplay]

I was playing Leblanc and couldn't use my ult while I was low on mana. I am not sure if this is what caused the bug as it seemed to fix itself a few seconds later when I was still 30 mana short for the ability (it did not use any mana). The ult was definitely available as I just hit level 6 moments before and it made the "oom" sound when trying to press the key.