I'm having the same issue with my 27" iMac. Apple case 100952587736. I too am a photographer who calibrates my monitors. Having the system reset to 100% brightness on wake or restart is a huge issue for me. Resetting PRAM, SMC, disconnecting all monitors and peripherals, and even wiping the hard disc and reinstalling the OS has had no effect on this issue. Starting in November 2019, a month before my warranty was up, I've spent countless hours and enormous amounts of lost productivity on this issue with Apple with no resolution in sight. My iMac has spent 10 days at the Genius Bar, where they saw the problem but had no fix. I was assigned to a senior Apple phone support person, who was initially very helpful but no longer responds. Her initial guess was a macOS bug that would be fixed at some point. That was two updates ago and still no resolution. We are now four months into this issue and I'm pulling my hair out. I've been a loyal Apple customer for 33 years, and I want my computer fixed or replaced. And I want it now! The silence from Apple on this issue is deafening. I expect more.