Disabling hardware acceleration fixed that for me, though I found that some webpages, mostly video-related ones freeze when you switch between tabs, so I would really like to see this feature change so I could leave hardware acceleration on.
Though, I'm fairly certain that this is a copyright protection system, if you have hardware acceleration on you also can't stream Netflix on discord, for example, in almost every browser.
But I never had this issue with youtube on other browsers before, so until then I'm using MS Edge to stream youtube videos on discord
The difference tho is that for me the Whole browser is just a black screen, not just the video:
Even when selecting the window to stream, the preview is all black:
Since I have a dual screen setup, I'm always tabbed out of the video window when I'm streaming it on discord so this adds a lot of headache to a simple "watch this cool meme video with your friends" task.