I´m having this issue, where no matter how I change the brightness, it does not change. It was working previously.
I have tried using the FN Keys (I can see the brightness level changing on the top left) as well as changing the brightness from the notification area, but the brightness does not change.
A mi me paso lo mismo. El problema fue por los ajustes HDR.
Entra en Configuracion - Sistema - Pantalla - Windows HD color - Configuracion HD color - Hacer streaming HD color - Cambiar la configuracion de calibracion
Aqui le das ha Restablecer configuracion en predeterminado.
Espero que te sirva, en mi caso funciono.