Seems webrtc webm files have no duration / bitrate added to them. Interesting. The solution is to make a quick copy (no transcode) and check the copy, which ffmpeg has added the duration/bitrate headers.

Seems like a hack, any proper way to do this @Alexander ?

ffprobe is my go to, but it also displays streams duration as empty. I'm afraid getting duration will require a copy made though ffmpeg. Then duration can be read. Then unlink copy.