Editing the template of a RibbonView in Blend for Visual Studio seems to work on my end. Can you please check if your dlls are all the same version?
If that doesn't solve the problem you can find the RibbonView template inside the installation folder of your version of Telerik UI for WPF in the Themes.Implicit folder. Then you can find the theme that you are using and locate the Telerik.Windows.Controls.RibbonView.xaml file.
You can read more about that here: Edit control templates
Let me know if I can be of any further assistance.
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