I have found and restored every barn find car including seasonal exclusive barn finds and Bamburgh castle’s barn find but the achievement process is stuck in 73% which probably means the game only counted 11/15 cars. This bug might get fixed BUT I REALLY REALLY REALLY REALLY hope that this doesn’t mean that I need to give up on my save and start a new game for a single achievement. 
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I’ve got all 15 cars and my progress is stuck in 89%.
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This is a known issue, my hard rocking amigos! …well…known by thousands of gamers… the game makers are trying their best to turn a blind eye…but it is getting more and more difficult for them to do so, thank goodness!
Here are some more threads for your perusal:
https://forums.forza.net/turn10_postst135603_Barn-Finds---Antique-Restorer-Achievement-Bugged.aspx
Good luck everyone…even the haters!
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Good to know at least I’m not alone