I have Forza 5 and i got 100% on two challenges yesterday, On the Horizon (Drive 500 miles in a Lamborghini) and The Drift Ring (Finish the Drift Ring rivals event having earned 25,000 drift points in one lap). I did all 500 miles in a Lamborghini and got 25,000 points in the drift ring, and the game recognizes it, it says 100% complete, but it is not giving me the challenge/achievement.
Hey just to let you know, there are still problems reguarding gaining achievements, microsofts teams are trying to fix the problem so im told by offical xbox support. Its not T10s fault there not unlocking. If there 100% complete they will unlock once the issue is resolved.
Having this exact problem with The drift ring challenge. Xbox support said it was nothing to do with them, despite the problem being system wide.