Gameplay - Xerath's passive gets disabled intermittently

I wish I had a reproduction method, but sadly "spam Xerath" is about all I've got. I've seen this off and on over several patches, but it's certainly more painful/noticeable if you're skipping a mana item.

Expected behavior:

• When Xerath's passive is lit up, he can auto attack minions, monsters, or champions for mana.

Observed behavior:

• The passive doesn't activate on auto attacking the above entities, despite not having full mana. (no mana given when it should be)
• The passive doesn't show itself on cooldown
• You can auto attack several times in a row and still not have it activate (not a fluke with first auto or something)
• "Randomly" starts and "randomly" ends (it doesn't persist throughout the entire rest of the match, but no known reproduction yet)
• Haven't particularly noticed it early game, it usually shows up mid to late game for me.

I'm a bit under the weather to remember perfectly which game (or to look it up now), but I know I've had this happen in one of my last three Xerath games.