Random Locking/Unlocking - Hardware or Software related?

I bought my phone on eBay for cheap since it’s easily repairable and is equipped with “Android One” , the seller had already replaced the display and Volume up/down and power button flex cable inside the phone.

About a week ago the phone started locking and unlocking seemingly at random, doing it almost immediately after I unlocked/locked it by pressing the button myself.

First I shifted towards a hardware issue, corroded contact pads, maybe some water inside the device. Since a factory reset trough the recovery didn’t solve the issue it was clear it has to be the hardware buttons.

The big confusion for me now is:

When I boot it into it’s recovery and control the menu with the volume buttons the power button should press itself like it would with the lock/unlock issue, but it doesn’t!

Is there any known android software issue, maybe an app which is trying to inject a routine in the background that’s doing this?

I don’t want to open my phone, replace everything, close it up and see it starting to happen again because it’s software related.