These specials have legacy mimetypes `application/x-font-ttf`/ `application/x-font-otf` for embedded fonts, likely due to using mkvmerge 88.0 (which had this regression). While `application/x-font-ttf` is recognised by FFmpeg, `application/x-font-otf` is not (used for `Montara Gothic`) [see libavfilter/vf_subtitles.c](https://github.com/FFmpeg/FFmpeg/blame/c57660fb18f058e8ead224e840b242d9c68fd3c4/libavfilter/vf_subtitles.c#L336), so this causes fallback to system fonts.
TLDR; change the `application/x-font-otf` to `font/otf` if you use ffmpeg for rendering.
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Moses35i
PsyPryss
ToishY