I have had a dead MacBook Pro (13-inch, 2016, Four Thunderbolt 3 Ports) sitting on my shelf for three years. I replaced it with a MacBook Pro (13-inch, 2019, Four Thunderbolt 3 Ports) which worked well but I sold it to a friend when I bought my current M1 MacBook Air. Unfortunately, the 2019 machine quickly developed a screen fault, which looked suspiciously like the fault that Apple recalled the 2016 model for. It means the screen just shows flickering purple squares and is apparently caused by the ribbon cable to the screen going faulty. My friend took it to a genius bar and was told it would cost over £500 to replace the screen. I was obviously embarrassed about this, so my friend and I hatched a plan to transfer the screen from my dead 2016 model to the 2019 one (the specs appeared to identical). We followed the instructions on iFixIt and were delighted when the computer appeared to work OK (after one initial kernel panic). However, our joy was short-lived when we realised that the screen was not very bright but worked perfectly in all other ways. After resetting the NVRAM, during the first part of start-up, the Apple logo was at full brightness but when the progress bar got to about 50%, the screen went dim again and stayed that way. We tried doing an SMC reset but that didn’t work and no amount of fiddling with settings helped.
Then the mystery deepened. He had Windows 10 installed via Boot Camp and when we restarted in that, the screen worked perfectly at full brightness. After much head scratching, the only thing I can think of is that the 2019 model supported TrueTone but the 2016 one didn’t. This would explain why Boot Camp works OK.
So my feeling is this could be sorted in software (possibly using Terminal?). We tried reinstalling Big Sur but that didn’t help. Does anybody have any ideas, please?