Pressing the power button usually results in the screen flashing on and turning right back off. Sometimes you need to double tap the button or time the presses just right to get the screen to turn on and stick.
Fortunately, I stumbled upon a solution to this........this assumes you are already using Xposed.
1) Download the "Xposed Additions" module
2) Activate the module and reboot your gear for it to take effect
3) Go into Xposed Additions -> Buttons -> Add new Key and then press the power button at the prompt.
3) Select the power key option that should now appear in the list and add a new condition for "Screen off"
4) Set the "Long press action" for "End Call".
NOTE: Long pressing the power button while in a call does not actually work. BUT you will notice that you can now reliable press the power button (just once) in order to succesfully wake the gear back up from the in-call screen.
I have noticed no other adverse effects after making this change but have only been testing it for about a day.