Is the power button faulty, or is the problem with iOS 9.1?

I recently bought an iPod Touch 6G (version 8.4), updated to 9.1, and to my surprise, the power button seems to be malfunctioning. Sometimes when I press it, the screen turns on and off, as if I were double-tapping the button. The problem persisted, so I returned it. They gave me a new one, and the same thing happened: version 8.4 worked perfectly, but after updating to 9.1, the screen/button problem returned. It seems very likely to be a bug in iOS 9.1, because it's unusual for two iPods to fail on the same day with the same problem, but not impossible. If you experience the same issue, please let me know, or I'll return it to the store. Thanks and regards.