GStreamer can't find these embedded fonts in my ASS subtitles

Related to https://gitlab.gnome.org/GNOME/Incubator/showtime/-/issues/144.
Sample videos here.

When testing with the GNOME "Showtime" video player app, even if forcing the presence of assrender, they render with the fallback font family instead of using the Matroska / MKV file's embedded font.

Correct rendering (Celluloid)	Incorrect rendering (Showtime with assrender bundled)

What is life without Comic Sans in my videos, but a tragic play by Elle Cordova?