--audio-format FORMAT ignored to download opus audio. Could not write header for output file #0

[Foxtrod89]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2020.01.01
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

youtube-dl --list-formats https://www.youtube.com/watch?v=4BDq_22PfWU&t=628s
[youtube] 4BDq_22PfWU: Downloading webpage
[youtube] 4BDq_22PfWU: Downloading video info webpage
[info] Available formats for 4BDq_22PfWU:
format code  extension  resolution note
249          webm       audio only tiny   51k , opus @ 50k (48000Hz), 31.25MiB
250          webm       audio only tiny   81k , opus @ 70k (48000Hz), 44.20MiB
251          webm       audio only tiny  133k , opus @160k (48000Hz), 74.26MiB
140          m4a        audio only tiny  135k , m4a_dash container, mp4a.40.2@128k (44100Hz), 87.77MiB
278          webm       256x144    144p   96k , webm container, vp9, 30fps, video only, 47.91MiB
160          mp4        256x144    144p  113k , avc1.4d400c, 30fps, video only, 40.44MiB
242          webm       426x240    240p  211k , vp9, 30fps, video only, 69.92MiB
133          mp4        426x240    240p  256k , avc1.4d4015, 30fps, video only, 94.52MiB
243          webm       640x360    360p  379k , vp9, 30fps, video only, 131.62MiB
134          mp4        640x360    360p  502k , avc1.4d401e, 30fps, video only, 183.92MiB
244          webm       854x480    480p  680k , vp9, 30fps, video only, 213.12MiB
135          mp4        854x480    480p  898k , avc1.4d401f, 30fps, video only, 333.53MiB
247          webm       1280x720   720p 1255k , vp9, 30fps, video only, 436.30MiB
136          mp4        1280x720   720p 1810k , avc1.4d401f, 30fps, video only, 686.32MiB
248          webm       1920x1080  1080p 2250k , vp9, 30fps, video only, 712.87MiB
137          mp4        1920x1080  1080p 3311k , avc1.640028, 30fps, video only, 1.29GiB
43           webm       640x360    360p , vp8.0, vorbis@128k, 400.22MiB
18           mp4        640x360    360p  393k , avc1.42001E, mp4a.40.2@ 96k (44100Hz), 266.68MiB
22           mp4        1280x720   720p 1140k , avc1.64001F, mp4a.40.2@192k (44100Hz) (best)

Description

I want to extract best OPUS based audio from Youtube.
For that i'm using:
--no-check-certificate --prefer-insecure --audio-format opus --extract-audio --postprocessor-args "-acodec libopus -b:a 64K" --external-downloader aria2c --external-downloader-args "-x16 -s16 -k1M"
It does work sometimes when you have m4a container as "best" audio stream, does work if you manually specify -format 251 which is overkill.
It doesn't work with for m4a_dash container as best possible audio stream.
I'm assuming because there is no way to demux opus from m4a_dash. Reported as bug due to switch --audio-format opus which will force youtubedl to get opus stream(best/worst???)

ENVIRONMENTS : Windows 7 x64, ffmpeg version N-71192-g3c91239, youtube-dl.exe
2020.01.01
OBSERVED RESULT: will not getting opus stream even with --audio-format opus
Could not write header for output file #0 - error pops up.
EXPECTED RESULT: will get opus stream.