Frostfang Gold Income

There's some kind of an irregularity I can't seem to pinpoint with frostfang's gold 'ticks' for casting a spell, etc.

For the past couple of weeks I've been convinced I was seeing things, but in this last game I played today I paid attention for it, and it definitely happens.

Usually on my first back I upgrade spelltheif's edge to frostfang, for the increased gold reward and stats, sometimes that back can happen pretty early, depend on how the lane is going.

When I first return to the lane with frostfang. I then cast a spell on the enemy. Let's say it's Morgana's pool. What happens? I only see +5 ticks. Not +10, as I should. This happens for little while. A couple cycles of the item at most. Then, I start seeing the +10 that I'm supposed to.

And it's not limited to just that. Occasionally I'll see a +20 when casting a spell then aa'ing in rapid succession (and it coincides with another +10).

TL;DR: Frostfang's 'Tribute' passive isn't working correctly.