Upon landing Katarina's Q on any number of enemy champions, only the first will reward gold. This is infinitely and regularly reproducible, simply hit more than 1 enemy champion with katarina's Q while holding spelltheif with more than 1 stack.
LostFr0st4 votes
Cannot trigger Tribute more than every 1.25 seconds.
That's likely the issue.
DM Me Zoe Lewds1 votesI don't know where you got that from, but the in game text is 'Cannot trigger more than 3 times per 30 seconds.' With other champions like Morgana W and every other support I've used, hitting multiple targets grant multiple instances of gold. Like Soraka's Q on both enemy laners.
LostFr0st1 votesThey added it in Patch 7.9
V7.9 NEW EFFECT: Can only trigger Tribute once every 1.25 seconds.
Yes, you're right that they should add it to the tooltip for clarity. If you log into your client and click the little bug-like button in the bottom right you can fill out a bug report on the tooltip having that missing from it.