I managed to reproduce this a couple of times. I'm not sure if it's strictly because of my masteries/runes or anything, but I'm including just in case.
I wanted to try out Golden Alistar in a custom game on Summoner's Rift (This worked with some other champs as well). I was using a Rune page of nothing but Flat Cooldown Reduction runes (Marks, seals, quints,and glyphs). My mastery page was 0/18/12 (Including the Bandit mastery, and the bonus CDR mastery).
I tested with and without buying items, and it didn't change a thing. I still got gold from the Bandit Mastery (1 gold from ally minions dying without you last hitting) and from last hitting minions, but no ambient gold until about 1:55.
I should mention that the minions spawned at 1:15, so there seems to be something out of sync here.
Or maybe I'm just clueless and this is how it's supposed to be, but I swear Ambient gold start and minion spawn are at the same time.