iPhone 5S - won't turn on without 'hard reset' but works perfect

Hi

I have a strange issue, I have an iPhone 5S with signs of water damage.

I have replaced the battery and screen,

The device in general works fine until the battery goes flat, or it is turned off.

When I turn it back on, the display does not light up at all although the phone is recognised normally in iTunes.

When I hold down both buttons it will boot up fine and work flawlessly.

The phone does not vibrate when I attempt to turn it back on so I believe although the phone is recognised fine by itunes it does not successfully boot fully until I hard reset it...

The phone otherwise works fine and I have restored it via DFU mode