Why is my iPhone screen replacement intermittently working?

Hi everyone, tried all my own research so far and have resorted to joining the forum! Hello :)

So basically, I had my Mum’s iPhone 6S recently replaced with a third party screen after she dropped the phone which had a slight crack running diagonally through it.

I thought for re-selling purposes in the future, it would be worth getting the screen repaired, and for ease of use.

However, I have encountered the awkward Apple issue. The screen decides to stop working every couple of hours, most often at the lock screen where a passcode is entered. The screen does also cease up sometimes on the home screen, and I have noticed certain gestures like scroll to the top and scrolling through pages feels a bit less smooth than usual.

I have tried the following:

1) Soft reset

2) Restore

*Have not tried DFU mode which is my next port of call, but want to know if this is worth the hassle?

Nothing seems to repair the issue. The phone is on the latest iOS 12.1.4 which I suspect is the culprit in this. Having read online from iPhone 8 and X screen replacement issues last year, it took an update from Apple to remedy the situation.

The service that repaired it for me has even taken the screen replacement out of the phone and put it in to another phone - he claims it all works fine. The problem lies in that the repairer hasn’t tested the phone for longer than an hour, when the problems start to surface. He suggested to me to restore the phone, or ‘wait for an iOS update in the next couple of weeks’, which is a bad suggestion in my eyes.

Any help?