Yeah, this is still happening, I have the last version of Edge and Big Sur (Macbook Air M1) but this only happens in Edge in Full screen. Not Only streaming but Full Screen in any kind of situation. You can check it disabling "View">"always shows toolbar in full screen mode" in the menu. When something dark is on the top you will see the line showing and hiding randomly. (Also streaming any king of content that reaches the top line of the screen)
It's not noticeable browsing because the webs are not always black, but the line is there. Also most people don't hide the toolbar in the full screen mode in Mac so they don't notice, only streaming or YouTube.
I hope you can fix that soon, it's a little distracting specially while streaming content.
thank you!