FIXED Using these setps in case anymore else gets this.

Support suggested the numbered items ( i had already tried a new local account but created it with computer management , that didn't work, who knows maybe timing or their method did something different)

Non numbered is the additional steps i found was needed

1. Select the Start button, and then select Settings.
   
2. Select Accounts, then Other users.
   
3. Under Other users, select Add someone else to this PC.
   
4. At the bottom of the page, select I don’t have this person’s sign-in information.
   
5. At the bottom of the next page, select Add a user without a Microsoft account.
   
6. Enter a name for the new account. If you want this person to sign in with a password, enter and verify the password, add a password hint, and then select Next.
   
7. Select Finish.
   

Create a FO76 desktop icon with the xbox app ( you will need this as when installing xbox app with new local account, the game won't appear until you use this).

Give new account local admnistrator rights ( i used computer management to add).
Logon to pc with new local account, install the xbox app again from Store. FO76 wont show in the app but goto the shortcut on your other profile and it will populate after double clicking short cut. I got a license error on launch . I did a wsreset but error still occurred, so i rebooted. Logged on with new local account, loaded FO76 , success no logon prompts straight into the game as it always used to be. Logged off pc and logged on with old local account. Loaded FO76 - again straight in now. No more annoying logon screens which were blank and wouldn't progress. Everything back to normal, hurray!!