I too am inclined to believe it is bugged, or not working at all. I have 140 gamerscore in Project Sunburst. I should think at least ONE of the achievements I have unlocked would unlock one more bonus.
But then, the achievement for reaching the rank of 'Duck Caller' in Chuck's Ducks 2 on Facebook didn't unlock that way for me either, and I'd been a 'Duck Caller' since around the time Crackdown 2 came out.
I think Seed and Ruffian need to pull their fingers out of their proverbial backsides and deliver this cross-platform stuff, or just patch in a solution.
"When you're in a situation like mine, you can only think in metaphors."
Always looking to boost games. Send me a message if you want/want to help.