I've recently got a new Macbook with Big Sur and I'm observing some different behaviour compared to my old Macbook with Catalina.
I have an external monitor connected as a secondary display and I have a video player streaming on full-screen on it. When I close the lid, on Catalina my video keeps playing on the external secondary display, however on my new laptop when I close the lid the external display turns into primary, and I'm seeing my laptop's windows rather than the video.
Any ideas what might be causing this?