The problem occurs if the display was replaced with a Chinese substitute
it was so in my case
Sigh, I have the same issue. I just upgraded my MacBook 2014 15" from Catalina to Big Sur. Brightness controls worked perfectly before the upgrade. Now keys F1/F2 do not affect brightness (but the keys themselves work fine), and I see no brightness control in display prefs or Control Center. I already tried suggested solutions like reset NVRAM and SMC, enter Safe Mode, with no success. I can't be sure that my diplay was not replaced because I bought the MacBook second hand, but I sincerely doubt this could be the reason.
I solved this by going to display preferences. click on the color tab. The color tab is located between the display and night shift tabs. under the color tab you'll see the option to calibrate. Click calibrate and it will restore all functionality to control brightness.
Same issue here on a 2017 Macbook Pro 13 inch 4 Thunderbolt 3 ports. had the problem in Big Sur and still have it in Monterey, downgrading to Catalina now hoping that helps. This has to be solved or recalled... I tried to reset NVRAM and SMC and all the fixes one can find online, nothing worked.
Confirm this problem exists on a MacBook Pro 2017 13" i5 (a1706) with touchbar. Have tried SMC reset, PRAM reset. Even reformat and re-install Monterey and same issue. No brightness control in display, Touchbar brightness control shows on screen graphic but no effect to brightness. Have tried Reboot, boot in to safe mode, and changing user. Also changing scaling and color prefs still no control on brightness.
I am a Computer tech of 40 years. Bought this MBP 2nd hand at a bargain price due to flexgate. Have replaced display.
System is configured as a Dual boot with Mojave and Monterey. Booting in to Mojave no issues brightness control work as it should. But in Monterey, nothing. Brightness is up full and it drains battery very quickly. Have raised a ticket with Apple support as this should not be happening. It maybe due to the display being changed but this would not explain why everything runs fine in Mojave. Definitely something is up with OS.