Unfortunately there is bug with the tracking of the barn finds. If you find several barn finds (like 3 or 4 for example) and ‘collect’ them all in one go after they are restored the achievement tracker does not work correctly, it will only count for one car and not for all of them. Each car should give you 6.67% progress achievement tracker.
Please fix this! It has been confirmed by several players at trueachievements.com
Seems to be bugged anyway. Only started tracking for me with the second car restored, which unlocked the Auto Barn achievement (“Find and restore your first Barn Find”).
I did not collect both at the same time and I haven’t used “fast restoration”.
Season did change to winter on arriving at the festival location, just before claiming the first car at the garage.
This has been noted and we are looking into this issue at the moment.
You can submit a ticket relating to this issue here.