When the TitleBarVisibility is Collapsed, the backstage close button is aligned with the top of the backstage adorner. The button should have a top margin applied. 
This reproduces when the BackstagePosition property of RadRibbonBackstage is set to Office2013, which is the default setting of newer Telerik themes.

To work this around, get the backstage close button and set its top margin.

 private void RadRibbonBackstage_Loaded(object sender, RoutedEventArgs e)
 {
     var backstage = (RadRibbonBackstage)sender;
     var closeButton = backstage.FindChildByType<RadRibbonButton>();
     closeButton.Margin = new Thickness(closeButton.Margin.Left, 15, closeButton.Margin.Right, closeButton.Margin.Bottom);
 }