FFmpeg hardcoded subtitles revert to default font if subtitle is html
Possible bug when feeding mov_text to subtitles filter. I can reproduce the issue in git master branch. I did not investigate further.

Workaround is to provide the subtitles filter the SRT subtitle stream present in your original MKV input (file.mkv):

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=&amp;H80000000,BorderStyle=3,Outline=1,Shadow=0,MarginV=20,Fontname=Liberation Mono'" output.mp4


Make sure stream_index is referencing the desired subtitle stream. See subtitles filter documentation.