After many things I tried on my iphone 6 plus I decided to open up the phone myself and slid a small piece of paper at the back of the power button and voila no more bugging from this silly bug. Probably a small piece of pin from the power button was touching with the mainboard of the phone triggering the switch. This piece of paper just did the trick preventing the pins from touching the mainboard. The power button works properly even with this piece of paper. In my opinion you should give it a go and saving loads of money for a fix if your phone's warranty has expired.