One of the issues with the old wording on Noble Brigand is that the when-buy trigger is part of the on-play text, and the on-play text should be activated each time you play the card, while text under the line is always active. "When you do X" phrases in the on-play text usually set up events that will be triggered in the future, such as the "when you gain a card" event in Cargo Ship. The "When you play this" gets resolved immediately, as it does with treasures that use that wording, but there's a remaining "When you buy this" in the on-play text. So there's a potential interpretation that says that every time you play a Noble Brigand, you're setting up a future "when you buy this" event that will be triggered if you ever bought the card.
Now ordinarily you can't buy a card you are already playing, but suppose that you've used Band of Misfits to play the top Noble Brigand in the pile multiple times this turn. If you then buy that top Noble Brigand later in the turn, then under one possible interpretation, you might get to do the attack multiple times. Now, are there people who are interpreting it that way? I dont know. Probably not, but the text was ambiguous and didn't follow the convention used in the rest of the game.
In the new version, the ambiguity comes from "What parts count as the attack?", and that's a lot easier to state in the FAQ than a clarification for what I wrote above.