Seems like there is a bug with video rendering or new "feature" that was not mentioned in change logs.
I use app OnTopReplica to grab video from page and show it on top of other windows. But about a week ago I faced a problem: video from youtube or twitch is not rendered when browser window is completely overlapped by other window. I'm sure problem is valid for any website with video tag. I tried to disable hardware acceleration or change some video related settings in edge://flags/, but video is still not rendered in such conditions.
I use Edge from dev channel. Same issue is present in canary channel. But beta channel works well. I managed to reproduce it on 2 PCs with Windows 10 and in Windows Sandbox. Attached video shows it using canary build in Windows Sandbox.