Chosen Solution
Whenever I try using the F5 and F6 keys to adjust the brightness, I can see on my screen that the brightness is being adjusted but the backlight doesn’t come on.
You may notice that the keyboard backlight may not allow adjusting with a small prohibition icon being shown in the adjustment window. This is because there is a setting in MacOS (on newer versions like High Sierra and Mojave) that is switched on to automatically turn off keyboard backlight in rooms that have bright lighting (There is a ambient light sensor for auto brightness right next to the webcam) That setting can be toggled off under keyboard or power on System Preferences. You can test to see if keyboard backlight turns on because of that setting by blocking roughly around the camera area and see if it switches on. Otherwise it sounds like bad keyboard backlight diffuser / film which usually only occurs from liquid damage on the keyboard.
