Hi Marc,
If you have one of our Metro themes active for your app (via ThemeManager.CurrentTheme), the backstage close button (circle with arrow) should be visible. It will revert to the standard File button only if using a non-Metro theme.
If that doesn't help solve your question, then please make a new simple sample project that shows the issue and email that to our support address. Reference this post and rename the .zip file extension so it doesn't get spam blocked. Thanks!