[GAMEPLAY] Xerath passive bug - won't trigger

I recently played Xerath, and right around mid game, I auto attacked a minion to try to regain mana via his passive. However, I auto attacked the same minion multiple times, only to have the passive not trigger, leaving me with no mana to join the teamfight that just erupted. I may have auto attacked another minion and have the same problem, though i don't remember exactly. The passive worked after I died, but had the passive worked when I needed it the most, I would probably have won that Xerath game...

I'm not sure what the circumstances were, but if I were to guess, it had to do with triggering the passive by auto attacking an enemy champion. Looks like I'll be stuck with until at least 5.20.