ME2 360 achievements have reset/disappeared.

Apologies if this has already come up in another thread; I did look around quite exhaustively to no avail but of course I have probably missed something.

I've just started a new ME2 game with an imported ME Shep and was surprised by the "Very Elusive" achievement popping up as I'd already got it (ages ago - I'm replaying).  I brought up the in-game 'mini-dashboard' menu to look at my achievements and saw that they had all reverted to the blank trophy graphic as though I had never gained them, but my gamerscore hasn't changed and when I look at the game title in the actual dashboard (that is, from the 360 dashboard proper and not in-game, if that makes sense) all previous achievements are there as they were before.   For info, I have changed my HDD and gamertag since originally gaining my ME2 achievements but am still using the same console, have not transfered any DRM, and have checked with other games and only ME2 is affected. It's not a problem (so far!) - I'm just curious to know if this has happened to anyone else?