I got 200/200 but the game isn't showing up on xbox.com and doesn't seem to have an achievements tab on GB either. Anyone other than me having this issue?

I'm having the same issue :( My first S-Rank too.

Since Microsoft just re-did their site, I think some of the achievement syncing has been messed up a bit.  Hopefully everything will be back to normal soon.

My GB profile hasn't updated since at least Wednesday

I can't seem to get the "Master of Disguise" achievement.  I've replayed the final battle and cycled my characters through all available costumes, yet no achievment...

" I'm having the same issue :( My first S-Rank too. "

I can't seem to get the "Master of Disguise" achievement.  I've replayed the final battle and cycled my characters through all available costumes, yet no achievment...

Achievement updates seem to have been borked since Microsoft updated xbox.com during the week in readiness for the imminent system update.

The dude that runs True Achievements has said the MS redid the entire back end on how sites can sync achievements from xbox.com and that there are issues with how DLC is now handled and he has had to rewrite  his scanners and still has to write a new algorithm to handle how DLC achievements are handled.  So i am guesing GiantBomb is having to deal with the same thing.
 
you can read his thoughts here http://www.trueachievements.com/n2064/scanner-update.htm