Found this on another thread with the same problem. Fixed up just right on my iMAC!
Hello Roro9,
Welcome to Apple Support Communities!
We understand that since updating your MacBook to Big Sur your screen keeps flickering when watching Netflix. We're happy to help!
Let's try an SMC reset.
How to reset the SMC of your Mac
You may also want to try an NVRAM reset.
Reset NVRAM or PRAM on your Mac
We also recommend starting in safe mode. Sometimes just starting in this mode and then restarting normally can resolve issues like this:
How to use safe mode on your Mac
Let us know if that worked for you.
Thank you for using Apple Support Communities.
Take care!