Halo 3 "Devastating" achievement won't unlock.

I recently played through the campaign in legendary to complete the Devastating achievement. When I finished my best mission times as up to well under 3 hours, but the achievement hasn’t unlocked. Is there a bug? Or did I miss a step?

Go to Career
Then stats
Then H3
Then Campaign stats
Scroll till u find : legendary beat time completed
if its under 3 hours its good, but when u look at it, it should pop

I got it the next time I logged in and loaded a game.