So there it is, my Instinct 2X (Tactical) doesn't honor backlight setting (5%) in sleep mode. I usually set the brightness during sleep time to just 5% in order to save my eyes being flashed in the middle of the night if I for any reason want to check the time. Instead, the watch just uses the same brightness setting (e.g. 60%) as if it is not in sleep mode.
Software version is 3.11