I have been experiencing this exact same issue over the past couple of days with the power button attempting to go to airplane mode/power off rather than simply turning the screen on/off. It's been difficult to even get it to do anything when pressed, but it seems as though pressing the button in just the right way (after a few attempts) will allow the screen to turn on after I let it time out and turn off. This whole issue popped up pretty suddenly, without having dropped it or spilling anything on it.
Interestingly, it will sometimes come on several seconds after I press the power button, leading me to believe it could also be software related. I've had so many issues with this phone - mostly needing to do factory resets after every major update to resolve instability issues, which is insanely annoying - and with my own troubleshooting, it's hard for me to be certain if it's a hardware or software issue.
I will try to see if a Sprint repair store can fix it, and while I'm skeptical, I'm also cautiously optimistic. It's nice to know I'm not the only one experiencing this problem, and that it's not a new issue exclusive to a new software update... thanks for posting this!