Glyphs overview: http://docs.telerik.com/devtools/wpf/styling-and-appearance/glyphs/common-styles-appearance-glyphs-overview Available in 2017 R2 Official Release.
The XAML Team did not find anything wrong with the way glyphs are implemented. This issue is now redirected to Microsoft for further investigation. More information can be found using the link below: https://connect.microsoft.com/VisualStudio/feedback/details/3107052/design-time-does-not-display-fonts-from-a-fonts-library