So, to reproduce the bug you must have the following settings in:
Settings/System/Backlight/Not During Activity:
Keys: ON (or after sunset)
Alerts: ON (or after sunset)
Gesture: ON (or after sunset)
(If you have the after sunset options, you must try it...after sunset!)
So, if you are laying on the bed, looking up, and bring your hand with the watch in front of your eyes, the screen may turn on or may not. In this position, if you press the Down button, the screen instantly lights on and off again. If you press again the Down button, the screen lights On again and it remains lighted for the amount of time it has been chosen in settings. So for some reason, in the first press of the down button, going from the watch face to the first widget, the light flashes. Only with the specific body state I described. If you are standing up, everything works ok.