You must have the layout set to "Disabled", where the video player is not supposed to be squared (the black pixels around the player is actually a bug, the whole thing is supposed to remain rounded). Try setting the layout to anything else (2021 is the closest to the current layout), and the video should be squared.

Edit: To change settings, pin the extension to the toolbar, then click on the icon to make the config window pop up. Here's how you do that: https://imgur.com/a/IIxddDa