Same problem here on a MacPro 2019 Big Sur 11.3.1 with 2 external monitors (so it does seem to be software related and not only happening on MacBook/MacBookPro). After waking from sleep or when waking the monitors, the resolution change in both my displays continuously. Each time, the colors and the resolution changes are different and it make take up to 1 minute until it stabilizes and become usable to enter the login password. I also get random flickering in the terminal (completely red display on some apps for 1 second or so). I tried multiple HDMI cables but nothing seems to work. Tried reseting NVRAM too.