An update was released a few days ago that patched this problem. I first heard of the patch over at trueachievements.com and the other day I finally went in and mopped up the last few levels which I had avoided and can confirm that this achievement is now fixed.
With as many other problems as there are on GFWL with glitches and achievements not popping properly it was nice to see one of them get fixed. I do believe this was in Microsoft's best interest as it seems silly to have a free game enticing you to purchase more games from them be broken.
Here's hoping they figure out why many of the other achievements for other games are so glitchy. I mean I enjoyed Where's Waldo but I was glad after the third time playing through on the easy difficulty the achievement that kept popping finally "stuck" on my gamertag.