I've used both TTF and OTF custom fonts many times in UWP apps without issue, but in this case, I have an OTF font that will not work in my app.
I have a test app that has one page with only the text on it, like this:
<TextBlock Text="Testing" FontFamily="ms-appx:///Assets/Fonts/customfont.otf#font name" FontSize="30"/>
The font file is located at that location in my project and the #fontname is the same one that shows in Windows Font Viewer.
I've tried converting the OTF to TTF, but it didn't change anything.
I've installed the OTF font on my Windows System and I can use the font in MS Word and other programs and it looks correct.
This is a professional font provided by a company that sells high quality fonts.
I've run out of trouble shooting tactics on this. Anything else I should try?