Same problem for me too with Big Sur OS using the Dell monitor model U2412M. I have two Dell monitors using Dell mini port dock, and only one monitor is having the flickering issue. After i disconnect the cable from the monitor to Dell mini docking port (cable being used to connect to the flickering monitor was the Basics 4K Mini DisplayPort to DisplayPort) and replaced by the direct connection via the USB-C type, problem fixed. So now the Dell mini dock is only being used for mouse, ethernet, etc, since both monitors are now connected to the Big sur mac book via direct connection using USB-C type-connection.