This has been an issue for years, 7 believe it or not.
In short, FH3 had two limited time exclusive barn finds that could only be unlocked by completing specific challenges on specific weeks. When the game was sunset in March 2019 with the final “Into The Horizon” set of challenges, these were permanent and are literally designed to last until the end of the century.
The only issue with these Forzathon challenges is the rewards are broken. They only award G-score, and not also the following rewards as originally advertised on the wiki:
As you can see, the wiki clearly states that completing the first and third challenges should unlock the Barn Find rumor. This is clearly not the case and hasn’t been for close to 7 years.
The big issue with these rumors being unavailable is that they’re factored into overall game completion. You will be stuck at 99% forever if you didn’t get these challenges completed in the small handful of weeks they were “enabled”
Clearly the developers tried futureproofing it by making sure they’re always active with Into The Horizon, but for one reason or another it’s broken and has remained unfixed for years.
I know this game hasn’t been officially updated since that final Forzathon challenge dropped and it’s probably abandonware, but if it isn’t I HIGHLY suggest making sure this is fixed and anyone with the game can 100% it properly. Even if that’s too much, just remove them from being factored into 100% completion. Thank you.