How to remove hidden games in my Xbox One library?

You cannot delete games from your account. Only hide them

Thank you for letting me know. I'm not sure why it would be removed from the library itself. Perhaps try going to My Games & Apps then use the filters at the top of the page to show your Hidden items. Perhaps it was hidden accidentally.

Yes there are more than one account on the console but I own all of the games as the other accounts are family accounts and all games get installed under my account. The only Games I have hidden are 5 Demo games which I have noted under the Hidden Games
element.

But none of that would explain why there are 20 more games in the My Games and Apps section and yet there are 20 less in the Full Library section. Surely the Full Library should have more as is called the FULL Library, not Partly Full Library, or Nearly
Empty library.

Yes you can view you hidden games. It's very easy too

1. Go to My Games and Apps

2. Go to Full Library

3. Select All Owned Games

4. Open the tab that says All Games and there you can select Hidden Games

There you have it, all the games and demos that you have but have hidden over time.

It has moved. Go into "My Game & Apps", then scroll down past Game, Apps, etc., until you find "Full Library."

From there, you should be able to access your Installed games, Ready to Install games, and Hidden ones too.