I used my own Chevelle and got it doing the shortest Reverse version of Maple valley, so it is something else. I think you just happen to achieve it that way. (EDIT: Considering that it seem it was confirmed that forzathon tracking issue is fixed, then probably you happen to have done it then and it worked. Same with other that are now doing it, it is just probably working now or at least refreshing faster).
Funny thing is: I shut down my Windows 10 PC like someone suggested in this thread, and suddenly ALL 3 Forzathons worked and progressed correctly even in the achievement tracker…
I started up the game again after seeing this tweet. Tried a 5 lap free play Maple Valley in Classic Street Muscle and also beat a rival. Neither of them worked. I still haven’t been able to unlock a single Forzathon goal.