repaired display not working after update to iOS 12

New display and new digitizer successfully installed. Everything was working properly.

After update to iOS 12, the display does not work anymore.

If one tries a hard reset the display works until is on but after having switching off the iPad the same situation appears again: the display is black (actually one can see the back light on but nothing else) and after an hard reset is working only until the iPad is switched on, when it goes in sleep or it is switched manually off, then it is not possible to switch the display on again (always only the back light is on).

Further, the display has now a big spot which is more black than the rest when the display is off and more white/bright of the rest when the display is on.

Anyone has an idea of what is the problem?

Thanks