When I call someone, after touching the green dial button I usually lift the phone to my ear immediately. But recently I become faster than my Samsung Galaxy S5. If I lifted the phone too fast the screen wouldn’t turn off at all, I had to move the phone away from my ear and put it back again to have the screen turned off.
Turns out, this is not a direct bug just a byproduct of a feature. I have turned on a Display feature that keeps the screen on until I look at the phone (under Display -> Smart Stay).
Turning off Smart Stay solved the problem, looks like Samsung needs to polish its software further.
Nope, it did not solved the problem, it is happening all the time.