When I press the guide button on the controller the game is displayed as “Forza Horizon 4 Demo” instead of just “Forza Horizon 4”. Naturally I have the full game.
In the “Recently played” games in the Xbox app in Windows the game is titled correctly though.
So, I think this is a MAJOR bug and all resources should be dedicated to it. Ok, just kidding on that last sentence.
edit: Ok, you can close this thread. Someone with better english than mine posted the same problem here: https://forums.forza.net/turn10_postsm972591_FH4-Full-Game-shows-up-as-Demo-on-Xbox.aspx