The content of RadBackstageItem is not stretched within the available area in the content area of the RadRibbonBackstage control. There is a gap between the title bar of the RadRibbonView control and the top border of the content area.
This reproduces with the Material theme, but it is possible to appear also in other themes.
To work this around, you can extract the ControlTemplate of RadRibbonBackstage and re-order the layout in order to stretch the content in all the available space. Check the attached project.