[MrMagic]

In all Kodi versions that I've used up until now, it was possible to use an external subtitle font. This worked by creating a directory tree "media/Fonts" in the userdata folder and copying a .ttf file to it.

I tried doing this using Kodi's File Manager, but it doesn't seem to work in the UWP version.

Is this a known issue already?

[afedchin]

Seems your did something wrong. It works on my test environment see screenshot https://imgur.com/a/5D7bX . here are corbelX.ttf were copied to LocalState/media/Fonts  folder.
Make sure that you add Home Folder as a source.

[MrMagic]

Thanks, I was missing the Home folder as a source and copied the new font to media/Fonts in the profile directory instead. It's working now!