I just unlocked Legend Clean Sweep after MANY failed attempts using every method mentioned here and everywhere else. Believe me, I tried EVERYTHING. Particularly a BRAND NEW SAVE starting with Rookie and proceeding through Pro, Champion and Legend. FOUR TIMES. No matter what I did the achievement wouldn't unlock as it was supposed to.
I did manage to get Legend to unlock after many failed attempts. I had a CLEAN SWEEP on Legend in GP save just before the final race. Many attempts. No achievements. After re-loading the save for a "what the hell" shot, I intentionally lost the last race and Legend popped. Never got clean sweep. Well, today I got it.
Thanks to "aka Kryptonian", he suggested I move my profile to a USB Flash Drive and try again. So. I loaded up my backup save just before the final race in GP Legend, won the race and the achievement popped! SO. Try that. Move your profile to a USB and attempt whatever achievement again that may have glitched on you. Good luck!!!