The Mascot achievement (Gotta Smash 'em All) appears bugged. I have all 200 mascots smashed, as confirmed by the journal (I have all rewards for smashing them all), there are none on the map, and the map region overviews show I have every single collectable smashed in every region. However the achievement is stuck at 99%, and the my stats page shows as 199/200. The bug possibly came from one mascot that I slid into incredibly slowly, which seemed to push the mascot without breaking it. It seemed to trigger it as broken regardless, though I did smash it afterwards properly anyway. That mascot is no longer on the map (none of them are), and there appears to be nothing I can do now to unlock this achievement.
Report bugs in the forza feedback portal.
I did, and they told me they could do nothing about it and gave me the link to post it here