DIAGNOSIS The Brightness Stabilizer or B.I.+ (Brightness Intelligence Plus) sensor is active. It's on the front bezel.
FIX Press the menu button. Go to Picture > Brightness Stabilizer. Set it to 'Off'. Then, go to 'B.I.+'. Set it to 'Off'. Manually set your preferred brightness in the Picture menu.
IF THAT DIDN'T WORK Ensure your room lighting is consistent. A flickering overhead light or a window with shifting sun can trigger the sensor. Cover the sensor (small circle on bottom bezel) with opaque tape as a test.
HEADS UP These features are for power saving and eye comfort. Turning them off gives you full manual control. You may see higher power consumption.