Spellthief's Edge and upgrades will not properly award two stacks of gold when hitting multiple champions with the same AoE ability.
For most champions, this is a non-issue as their AoE abilities hit multiple times ( 
), but for others this presents an issue ( 
).
I have tested this with Lux E and Taliyah W in bot games as well as in practice tool with dummies.
It's been like that for a while. I'm pretty sure they just changed it, though I could be wrong.
EDIT: Here's the info.
V7.9
New Effect: Can only trigger Tribute once every 1.25 seconds.
Removed: Killing a minion disables Tribute for 12 seconds.
New Effect: Killing a minion stops you from triggering Tribune for 6 seconds per minion slain.
New Effect: Quest.
Quest: Earn Gold 650 using this item and upgrade to Frostfang item.png Frostfang.
Reward: Tribute is upgraded into Queen's Tribute.
Queen's Tribute: Spells and basic attacks against champions or buildings deal 15 additional damage, grant Gold 15 and 50% bonus movement speed for 1 second (additional stacks extend duration). This can occur up to 3 times every 30 seconds.
Not a bug, working as intended.