So i was playing yesterday and never noticed this bug until i jumped on today, it happend 2 times with the Hoonigan Belair, the drivertar is a friend of mine, and it was his car both times, this is campaign by the way. i also talked with forza on twitter and they told me to try and restart the race after it happend, first race was ok, 2nd race was bugged, but it skipped me to the 3rd race so it didnt let me restart it, 3rd race was ok, and then the last race it was bugged again, i completed the race with the bugged drivatar car, then i restarted the race after and the drivatar car was ok. this has nothing to do with the car im driving, this is a drivatar car that is not rendering into the game, there was atleast 4 Hoonigan Belair in the game and it was only one out of 24 cars in the race that was bugged. same drivatar and same car both times.
Screenshot after 2 hours without the bug when it happend again :
bumping to see if someone else has got this bug or not.
Update : so i played for like 2 hours straight without a bug and then it happend again, and it the same Hoonigan Belair got bugged, this time it was another drivatar that was driving the car, and its only one car. restarting the race after it was completed fixed the “bugged” car and it showed up like normal 
i also tryed to run it on lower settings to see if it was a “bug” with amazing graphics that made the car not rendering but it didnt help at all.
Thanks for the info. We are looking into it from the tweet you sent.
Amazing ! thank you, great work guys !