That's how mine works,which is a great improvement as before 4.4.4 i couldn't get the blank screen to come live so had to wait for the other person to end the call and also could never use the keypad on menu's etc.

 

That sounds like a bug. The only situation I could think of where the phone would lock (so that it does not automatically turn the screen back on and start recognizing touch input) is if you were to accidentally hit the power button while on your call, thus telling the screen to turn off and lock. The phone should consider a phone call as the phone being unlocked and in use, so it should not lock out on you.

 

Before 4.4.4 update my phone never reawoke after removing it from my ear and at the time i couldn't liven it up by the power button so i'm pleased i now have this power button option when i need it.