The change was made to the 650S due to it being exactly the same as the 650 that we already had in the game when the 650S was made available. It isn't a bug, they specifically changed the model after the community complained it was an identical car because you couldn't drive it with the roof off originally.
Rules of troubleshooting:
1) Have you tried turning it off and on again?
2) Is your Internet working?
3)
https://support.xbox.com/en-US/xbox-live-status < Check here for service issues.
4) If all else fails open a service ticket and/or forum post.