Have not heard of this occurring. What is the gamertag that is being affected by this? Can you let me know specifically what achievements are missing?
Can you also clarify what console you are playing on?
You may want to try deleting and recovering your gamertag. Delete Xbox Profile | Download Xbox Profile | Move Xbox Profile & Gamertag - Xbox.com Be sure to select delete profile only.
Unfortunately, we're unable to push achievements to the account as they are linked to your Xbox Live account. If the achievements are reporting inaccurately after deleting and recovering your profile, you may need to contact Xbox Live Support Welcome to Xbox Support - Xbox.com
We'll continue to monitor the matter and forward it to the proper channels.