Good catch. I was already testing that and after disabling hard acceleration, I'm seeing less black windows and when it appears it's for a split second so not as noticeable as my gif above. Dropped frames are still a thing on YouTube but it's not much noticeable anyway. But pretty sure both issues are connected. It's having some issues with rendering. I haven't used any Chromium browsers regularly for the last 3 years so not sure yet if it's a chromium related issue or Edge exclusive only but I think it's the latter one.
Actually, I got that idea from Brave only showing black screen. There's another suggestion you might try if you check the link.There have been other reports, mostly re: videos, on Brave but not as many overall as on Edge.
I'm a regular in the Brave Community in the hope that they'll improve core features, which have some issues. Maybe I shouldn't hold my breath?