Hello All! I had this problem with my own account. I had an original account that was then was moved to Microsoft and it was showing play Demo, even though on my other two accounts that have the game pass could play. I noticed that my Xbox icon did not seem to load in the top left corner of the launcher. So when I switched to one of those different accounts and switch back it was fine. It showed up as I could play and opened just fine. I tested it by closing and opening it again and it was completely fine with no problems and haven't had any problems sense. What I did was that when switching accounts, I did it in the launcher. I don't know if it is something wrong with the login system or verifying you own the game. But you can try that.
JoiningToast
Hello All! I had this problem with my own account. I had an original account that was then was moved to Microsoft and it was showing play Demo, even though on my other two accounts that have the game pass could play. I noticed that my Xbox icon did not seem to load in the top left corner of the launcher. So when I switched to one of those different accounts and switch back it was fine. It showed up as I could play and opened just fine. I tested it by closing and opening it again and it was completely fine with no problems and haven't had any problems sense. What I did was that when switching accounts, I did it in the launcher. I don't know if it is something wrong with the login system or verifying you own the game. But you can try that.