Nothing like that has happened to me. I also have been connected to Wifi the entire time though. Is there any change you had a sketchy connect to Xbox Live when playing the game and they didnt unlock?
While it is not typical, some games do require an internet connection for the achievements to work. Just trying to see if that is the case with this game.
On a side note, Xbox.com has been a little sporadic for me the last 24 hours. Could be related. It seems sometimes when I try to load the site, it pops up with an error from Xbox, then several more reload of the site also give the same error, then randomly the site is fine again a minute later. Probably unrelated, but it is food for thought.