A long time ago, I posted a forum about all the cars from the Pre-order Car Pack being added to the garage for no reason whatsoever, and I was unable to use them as there was no marketplace data.
Several months later, and when I try to access one of the pre-order cars now, it comes up with a message saying “This is a DLC car that you have not purchased from the Marketplace.” It does give me the option to remove that car from my garage if I wish to.
I never pre-ordered Forza Horizon 3 whatsoever, although I did buy the game on release day, so I have no idea why the Pre-order Car Pack cars have been added to my garage, and yet it won’t let me use them.