The RadBackstageItem is a HeaderedContentControl and it does not have a StringFormat property, that is why when you set the Header like you showed in the your post the "Estimate Information:" is not displayed. If you want to display a StringFormat you need to bind your Header without setting StringFormat in the binding and also set the HeaderStringFormat property of the RadRibbonBackstageItem. You can observe that same behavior with the MS TabControl and its Tabitems.
Currently the HeaderStringFormat property of the RadRibbonBackstageItem is not working and even setting the HeaderStringFormat will not work. That is why I have logged that bug in our system and you could use our feedback portal to track its progress. I have also update your Telerik points as a small appreciation for your cooperation.
Please excuse us for any inconvenience caused.
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