[1] ffmpeg Documentation (2020-09-30T16:23:40.000Z, 2020-10-01T08:01:33.007Z) https://ffmpeg.org/ffmpeg.html

[2]

FFMPEG ignoring attempts to set user agent or other headers - Stack Overflow (2021-06-11 12:03:20 +09:00) https://stackoverflow.com/questions/59810888/ffmpeg-ignoring-attempts-to-set-user-agent-or-other-headers

It seems that FFMPEG only sends the custom headers if the input m3u8 is accessed via http/s, not if it is a local file.