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.
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