Try doing a system reset (hard boot) as below:
1 – from the Dashboard, out of all games, hold the X button on the front of the console down until it shuts the system down.
2 – Disconnect the external power from the wall plug (or at least disconnect the wall-side connector on the power pack itself.
3 – WAIT until all power drains from the external power pack (no light, either white or dull orange showing on the pack), and disconnect from the console itself.
4 – After about three minutes, reconnect the console side of the pack to the Xbox One and then the power supply to the electrical outlet.
5 – Restart the console in the usual manner, i.e., either with the controller or a short push on the X button on the front of the X1.
6 – WAIT until the console self-checks and fully boots to the Dashboard before attempting anything.
If you don’t disconnect the external power pack and let the power completely drain out, the system is still retaining “old” stuff and not resetting.
You will probably receive an update to Forza Motorsport 6 today (already released regarding the new car pack) so let that download 100% before attempting to play the game.