That sounds like the power button itself may be getting a bit worn out. I know that my personal OV I sometimes have issues with it. When I long-press for the power menu, often it will appear then the screen turns off. I did some testing with the donor phone to be sure it wasn't a software issue, and had no problems on it running the same build.
I'm pretty sure it's not a power button issue
Just getting so used to holding it a certain amount of time that you have it timed near perfectly
And there is a split second window when you hold it long enough for the phone to *tick* and the menu to pop up. but too short that it turns off the screen.
I did this very often on my old phone and when i got replacement phone it continued.