FFmpeg hardcoded subtitles revert to default font if subtitle is html

Here’s how I’m using ffmpeg to hardcode the subtitles:

ffmpeg -i tmp/input.mp4 -vf "subtitles=tmp/input.mp4:force_style='OutlineColour=&H80000000,BorderStyle=3,Outline=1,Shadow=0,MarginV=20,Fontname=Liberation Mono'" -avoid_negative_ts make_zero tmp/output.mp4

which works fine if the subtitle is plain text but if it has html tags in it, then it reverts back to using system’s default font. <i> hello </i> for example causes problem.

I’ve tried cleaning up html tags and then using -vf "subtitles=cleanedsubs.srt:..." but it doesn’t work even on plain texts.

How can I force same font even on italic / bold / any other forms?

Extra info

Running:

ffmpeg -i tmp/input.mp4 -filter_complex "subtitles=tmp/input.mp4:force_style='fontname=Liberation Mono'" -f null -

Shows:

[Parsed_subtitles_0 @ 0x55ad579abd00] Using font provider fontconfig
Stream mapping:
  Stream #0:1 (h264) -> subtitles (graph 0)
  subtitles (graph 0) -> Stream #0:0 (wrapped_avframe)
  Stream #0:0 -> #0:1 (aac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
[Parsed_subtitles_0 @ 0x55ad579aa3c0] Shaper: FriBidi 0.19.7 (SIMPLE) HarfBuzz-ng 1.7.2 (COMPLEX)
[Parsed_subtitles_0 @ 0x55ad579aa3c0] Using font provider fontconfig
[Parsed_subtitles_0 @ 0x55ad579aa3c0] fontselect: (Liberation Mono, 400, 0) -> /usr/share/fonts/truetype/liberation/LiberationMono-Regular.ttf, 0, LiberationMono
[Parsed_subtitles_0 @ 0x55ad579aa3c0] fontselect: (Liberation Mono, 400, 100) -> /usr/share/fonts/truetype/liberation/LiberationMono-Italic.ttf, 0, LiberationMono-Italic
[Parsed_subtitles_0 @ 0x55ad579aa3c0] fontselect: (Serif, 400, 100) -> /usr/share/fonts/truetype/dejavu/DejaVuSerif-Italic.ttf, 0, DejaVuSerif-Italic
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                                                 problem                                            

ffmpeg -i file.mkv -map 0:a:0 -map 0:v -c:v libx264 -c:a aac -vf "subtitles=file.mkv:stream_index=6:force_style='OutlineColour=&H80000000,BorderStyle=3,Outline=1,Shadow=0,MarginV=20,Fontname=Liberation Mono'" output.mp4