Similar problem- 2017 MBP, updated to Big Sur weeks ago, been enjoying it until yesterday after a reboot the display became full brightness and could not be adjusted whatsoever. Brightness not responding to the adjustment bar in the touchbar + automatically adjust brightness option missing in system preference. Had to use a third party software to manually decrease the brightness.... anyone has any idea why and how to fix this? I hope it’s a software glitch but not something hardware wise