Hi guys,
I'm from the Xbox360 part of the website. Just so you know, we've been discussing the issue for a month now (the DLC was released on March the 18th for us) and have come up with a 100% reliable explanation (didn't read your thread, so you might have done the same).
Anyway, sorry to disappoint you, but EA WILL NOT FIX THE PROBLEM.
If you want, you are welcome to join us in the 2 dedicated threads:
The explanation:
[DLC] Achievement glitch explanation - Xbox360Achievements.org
The discussion thread:
Are the achievements glitched??? - Page 7 - Xbox360Achievements.org
It would be great if both side of the community shared their info.