Hi there! Same problem on WidgetKit complications extension. On Watch Series 10 it doesn't works - complication become blank, on Simulator however it does.

I had an idea that me and OP have this issue because we use shared App Group container. So I tried this workaround: Copied all fonts from shared container to Documents. Then install them from Documents. It didn't work. I suspect that even if we include fonts to the Bundle, skip UIAppFonts, add them using CTFontManagerRegisterFontsForURL - they will not work. 😔

I found a workaround! Will publish an open source solution when I have time. Please, poke me if you need it.

I wish we had access to that component that's used above keyboard in Safari or Notes.