There is indeed a problem with the achievements and gamerscore.
I unlocked a few achievements, went to the dashboard, the gamercard was there with the icons of the unlocked achievements.
When I turned off my console to pick up my son, and came back home almost 90 minutes after shutting down the xbox, I turned it back on. The achievements were gone, but my new gamerscore (old GS + gamerscore from DS:I) was still there.
Solution as always: sign out your account and recover gamertag to get your 'old' gamerscore back until this problem is fixed.
I updated the achievement guide accordingly.