This may be related to the Knock Code functionality, which allows you set a pattern of 2-8 taps (knocks) to wake up and directly unlock the display.
When enabled (Settings > Gestures) Knock On has 2 functions:
1. Screen on: When the display is off (black, in standby mode), double-tap on the same position of the display and the display will wake up.
2. Screen off: When the display is on, a double tap puts the phone in standby mode.
When this function is set, it disables the Screen on, because waking up the display is done by the Knock Code function. Screen off still works; this is not influenced by Knock Code.
Settings menu shows greyed out Knock On function when Knock Code is activated:
