There are some units in the wild where the brightness is not available through Android. In this case the brightness is just handled by the main board in your unit.
I´m fighting with it also since a 1 year now and even with Tasker there is no way to adapt this by Fcc.
The other units are working fine, which have a different CPU/MCU combination.
You can try the following:
Connect with ADB to your unit and try:
adb shell settings get system screen_brightness
Then try to change the brightness and observe if the brightness changes:
adb shell settings put system screen_brightness 200
If nothing changes, just try the first command again and you will see that Android "tried" to set the brightness, but it is not set by the main board.
Check with Logcat if you can see the logs if you change the brightness manually. May be one is able to find a way to get this working somehow.