The Reason is a GHOST Epic Games account has been created, and you no longer get the activation screen.
To sign into that GHOST account you need to set it up first.
Login to Epic Games using your Xbox Account, EPic games will ask you for Name and Details to setup account.
Once logged in you can now navigate to the "Account - Connected Accounts" area and Select the "Disconnect" for Xbox.
Once this is done ensure you log out.
Be careful of the email addresses used here, using the same email address caused me problems and required an email address change.
If your xbox and and proper Epic Game account already use the same email address, and you are setting up the Ghost account prior to disconnecting, then you could find that your Proper Account gets screwed up.
Once you have disconnected the Ghost Account you are free to start Fortnite again and this time correctly linking/activating EG, using your proper signed in Epic Games Account.
This process probably goes for all platforms, I know it does for Nintendo, and pretty sure for Playstation.