this didn't unlock for me and others. i completed every scenario on different difficulties from very easy to very hard.
if you want to avoid the glitch, never switch the difficulty after you started the scenarios.
PlattfussGDfan found a solution for anyone, who already has the glitch. he wrote:
Err...I...errr, did the whole thing again!
Changing the difficulty is 100% causing the bug, so -> go to Colin McRae scenarios and choose a difficulty and save (91 or more for both achievements) -> go to settings -> profile -> save settings -> first option (progress reset) and do it all over again and NEVER touch the difficulty settings.
This will delete some of your data however...all Colin McRae scenarios, all historic championships, WRX Championship, My Team Rally/Rallycross career (but nothing in Garage or Crew), saved setups and everything saved in custom championships.
Or you wait for a patch.