Gamble + Nobles (Draw 3 cards)

[klugerama]

Game 88623390: Klugerama's turn 13.

Selected Gamble. Drew Nobles, played Nobles as draw cards (+3), drew platinum, but game would not allow spending platinum. Recurred on turn 14 with copper; game wouldn't allow spending copper.

[Ingix]

That's not a bug, it's the correct behavior.

The Buy phase has kind of 2 sub-phases. First you play Treasures or do other things to generate coins (like spending Coffers). When you start buying something, the second sub-phase starts, where you spend the coins (or Potions) to buy things.

You can't go back from the second sub-phase to the first sub-phase (unless you have an affect that lets you go back to the Action phase explicitly, like Villa or Cavalry, where another Buy phase starts afterwards).

This is spelled out in the rulebook of the 2nd edition base game as follows, at the end of the explanation how the buy phase works:

You cannot go back and play more Treasures after buying a card; first play Treasures, then buy.

So, that's why could couldn't play the Platinum/Copper in your game. you had started buying things, in this case the Gamble event, so could no longer player Treasure. The base game does not contain Events, so its rulebook doesn't mention them, but it applies equally to them as well.