Yesterday I had the problem of not seeing any videos in the Stream WebPart in SharePoint Online, even though everything was configured correctly. Today the same phenomenon in Power Apps when using the new Stream video control.
I went into it once and want to share the result:
The display of videos in Power Apps and in SharePoint Online runs as long as the Stream Web Client is not opened in the browser instance. This prevents the videos from loading. To see the videos again, the web browser instance must be closed and a new one opened. Then you can see the videos in SharePoint Online and in Power Apps again.
I guess a CORS problem. I was using Chrome Version 80.0.3987.162 (Offizieller Build) (64-Bit)