Why does my screen look distorted?

Recently I sent my iPad to be fixed for broken screen (the LCD was intact), they separate the digitizer from the lcd and put a new digitizer on and keep the original lcd.

They did a good job fixing it, but after a day it began to have a distorted screen when I wake up the iPad. It goes away and function normally after a couples of seconds and sometimes for longer. When it goes away the screen looks and functions completely normal, the problem only occurs when the iPad is left a couple of hours not used.

It’s a odd problem, could it be loose cable fitting? Have anyone of experienced the problem before?