Pretty much as the title says. All the prize cars i’ve seen are available in the autoshow.
A similar item “Barn Finds - Barn Find cars from FH4 are labeled as Barn Finds in FH5 when auctioned” is in the Known Issues list, but this one is not. Please help the Forza Support team log it by submitting a ticket:
https://forums.forza.net/turn10_postst168526_PLEASE-READ--visit-Forza-Support-and-Submit-a-Ticket-before-posting-here.aspx