Yeah, sorry for that :disappointed: At the moment it is the intended behavior. There are users with old touchscreens (hardware revision 1.0), which do not support dimming (see 
#91). To stay usable, the brightness value shouldn't fall below 128, which doesn't leave much of range for the actual dimming. Long story short, in order to support both screens, we decided to sacrifice the dimming option. However, we have a plan to add this option to the settings and allow users to choose which touchscreen they have. In the mean time, you can extract the file "lights.rpi3.so" from the old release and replace it in 0.4.4. It will bring you the desired behavior back. Sorry for the invonvenience.
