iPhone restarts when HOME button held down, power button unresponsive

I have a donated iPhone 6s, which after many replacement parts has been brought back from the dead (screen, button cable).

I have repaired hundreds of iPhones, and never met a problem that couldn’t be solved by replacing parts.

It works wonderfully, but there are two quirks that I am yet to work out:

1. The power/sleep button is unresponsive, despite replacing the button cable.
2. If the HOME button is held down the phone restarts, and sometimes goes into a boot loop.

I have:

• Replaced the button cable

* reset it

• wiped it
• managed to get it into DFU mode (which baffles me, as I thought the power button didn’t work… maybe it works but doesn’t respond correctly?) and updated it

Anyone fancy making any suggestions? It is not HUGELY important, but I am curious as to what might have happened here!