Dragon Age II achievement glitch

Gamertag: Xenosaga Freak

I played the Sebastian DLC in June and for the life of me, I couldn't figure out how to unlock the Avenged achievment. You're supposed to automatically unlock it at the end of Act I and for some reason, it had never popped up. The other achievements for this DLC have unlocked properly except that one. I tried clearing the cache and deleting and redownloading the DLC, but nothing changed. I gave up on it after that.

Now that the Dragon Age Keep has come out, I checked my sync data as well as my Xbox Achievements list and now it's listing the Avenged achievment unlocked. But now, Mage Hunter, which I've unlocked in one of my first playthroughs is now locked? I'm so confused how this happened. Avenged was the last achievement I needed to earn for completion. I'm more than sure of this because I remember being so frustrated about it not unlocking. Now I'm missing an achievment that should have already unlocked. Is there some sort of explanation how this came to be? Do I have to complete the game again to unlock this one achievment that should have been unlocked already? @_@ I'm so utterly confused.