Powering back on by itself after shutdown.

Hi,

I have an iPhone 6 that was draining its battery very quickly. I first changed the battery in hope this would fix the problem, but it still drained very quickly. I then replaced the Tristar IC. This fixed the battery drain issue. Although now when I power off the phone, it stays off for about 5 seconds then powers on by itself. I also notice that when I attach a charged battery, the phone powers on without pressing the power button. I am unsure if this was a problem before changing the tristar IC. No cable plugged in to the phone, power button flex is functioning fine, iPhone 6 running iOS 11.4. Any help would be greatly appreciated.