I've only noticed this at game start, before minions spawn. Perhaps because that's the only time I actually really pay attention to it.
While waiting to help jungler with Blue, I usually throw a spell or two in brushes that enemies may be guarding their jungle in to get a bit of early Spellthief's gold. I have not quite found what makes the difference, but I would not always correctly recieve gold, while the Spellthief's item does lose a stack and all other things that trigger on spell hit do go off (Aery, Manaflow band, etc.).
I've encountered this bug on Karma, Anivia and Brand so far. Every time this has happened, I didn't actually have vision of the enemy I hit.
Here is a [Screenshot](https://i.imgur.com/J9TuJcH.png) after the bug occured on Brand.
I thought perhaps I wasn't close enough to an ally to receive gold, but then I shouldn't lose a stack, should I?