Small black square in top right corner
I believe this is a bug with the menubar transparency in Big Sur.
I often get the same issue often when I switch between spaces with darker and lighter desktop images, as the transparency/font of the menubar adapts to your desktop image. For example, a darker desktop image shows the menubar with white font, but a lighter desktop image shows the menubar as black font.
If you don't care much about the menubar transparency, you could reduce transparency in System Preferences&gt;Accessibility&gt;Display and enable Reduce transparency. This will then keep the menubar as a solid grey menubar and hopefully stop the issue.
Another solution would be to click on one of the menubar options in the affected area or switch between different desktop spaces, and let the UI processes recalibrate.
You could also kill some UI processes in terminal to get it to reset, but I often find the terminal command killall Dock is a safe one and does the job fine.