I beat the game yesterday on Normal Mode, and have been going back through it on Nightmare mode. Twice now the game has granted me the Heroic Mode achievement while taking away my previous achievement for Normal Mode. It's happened on the Skeleton King (Achievement: The Mad King's End) and Maghda (Achievement: The Coven Overthrown).

Is anyone else having this problem? Or do I just have to go back and beat the bosses again?