I remember I had the same issue when I played fortune island - it took a while for the achievment to pop up, but unfortunately I don’t remember exactly what I did when it finally unlocked. Maybe try redoing the race in a different car class, or jumping back and forth between the mainland and fortune island and doing some stuff on the mainland before redoing the race - that’s (roughly) what I did while waiting for the achievement to unlock.
I did this in the wrong Fiesta originally (DOH!) but unlocked it when I used the right car … it does sound like you are using the right car (#43 on the side of it?)
Ive tried the race several times and short blueprints and it still has not unlocked yet. Ive noticed one thing the RX43 i have is a epic car, not legendary, could this be the problem?
I tried to remove the car and buy it again from festival or auctions, but it still shows up an an epic car. Ive tried to race several times even last year.
Did this ever unlock for you ? I’m having the same issue. Late to the party but yeah, doing it legit and the right car EXCEPT for that mine is also Epic not the old Legendary version.
EDIT : fwiw this did eventually unlock, after a couple restarts iirc, if that’s what triggered it, who knows…