Xperia Z5 :: Phone Brightness Control Bar Out Of Control Under Adaptive Brightness

With my old Xperia Z1c, when adaptive brightness is on and the environment light is darkest, the screen brightness may be adjusted to, let's say, 0.  Even you drag the brightness control bar to leftmost, the dimmest brightness you can get is still 0.  But in Xperia Z5, you can go dimmer than that with the control bar, and this dim cannot be achieved using the same control bar if you disable the adaptive brightness.  If one component cannot do something in manual mode, should not be able to do that thing in automatic mode either.  So I believe that this is a bug.