While I never experienced these issues on either my Desktop or my Surface Pro running Windows 8.1, it does appear Microsoft has added YouTube.com to the compatibility list as it now renders as Document Mode: 10 if you check F12 Developer Tools.
Previously, and correctly in my opinion, it rendered in Edge mode, I liked that it was using the HTML5 player for most videos as well as allowing it to go actual full screen, but not anymore...
I just verified this by opening it on my Surface Pro, which I hadn't used today and checking F12 and it said Document Mode: Edge (default), then checked a few minutes later and it had switched to Document Mode: 10 (default) so it must have updated its compatibility list from the server.
I suppose I can just manually disable the compatibility list for now...hopefully they get this sorted out.
Unless someone knows how to force it to Edge mode without disabling the compatibility list?