This the best explantion for the problem from "HVAC Techie" on trueachievements.com
"Nothing wrong with a little wishful thinking. Honestly it's likely a very easy fix, it's just that they likely haven't given much thought to supporting where the files are uploaded. A "failed" upload (on BC) yields this: http://i.imgur.com/mqaINkY.png Even though it's technically uploaded to the File Browser, and other people can download it and see it in their recommendations, it's still listed as pending. So it's still storing them, it's just missing some authentication check. So either fixing that, or just making it so that just pressing x to upload something pops the achievement - changing achievement criteria isn't hard, plenty of games have done it, and a Microsoft-published game could certainly manage it.Problem is, nobody involved with anything Halo will say anything about the achievement when prompted. And though I assume it was 343 that did the BC porting, that's just a theory. But we can't even get a yes or no out of them involving anything."
pls 343 fix it