I am having the same problem, although I am downloading audio only. Everything looks good and the progress percantage climbs to 100% then nothing is saved (a file is saved but is 0 bytes).
Until 9-12-2021 I in Web RTC could play video in pot player but now when the video ends it tags but will not mux in this mode pot player says the file exits but does not seem to have any video so I changed over to HLS and pot player can than play the file normally again.
Also it does the same as Chris explained in Web RTC 0 bytes.
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