iPhone 6 black screen after screen change and iOS 12 update

Ok, so I run an iPhone repair company and have fixed prob 2000+ phones, so am not a novice. A few days ago I replaced a screen on an iPhone 6, battery disconnected first. All worked fine. The customer came back with the phone a few days later reported that she was using it had 70% battery all was fine and whilst sending a text the screen went black (she updated to iOS 12 less than 24 hours before and had downloaded a few apps)…. my first thought was to change the screen, so proceeded with replacing screen and once again disconnected battery first. All worked fine again. Told the customer, she came to collect it… by the time she got here, screen was black again. So next I tried to put it into restore and tried to update it on iTunes, it failed, tried on reboot it failed, tried to factory reset it and it failed. It got quite hot. I then tested the first screen I put on it on another iPhone 6, screen was fine on that phone. I then disconnected the 2nd screen (battery disconnected first) tried that on another iPhone and all worked. I then tried a 3rd screen and a brand new battery (that was tested first) on the phone, wasn’t recognised by iTunes or reiboot, tried putting it into restore as sometimes I have found this will force phone to be recognised…. wasn’t recognised and now appears completely dead…. does anyone know why or how this has happened? Can it be repaired?? If so what is my next step.

Any advice or help with this would be much appreciated.