Hi
Try restoring you phone to see if it’s a software issue as if it is it should go away after a restore
If that doesn’t work try taking the motherboard out and inspect for damage or discolouration as it could be a board related issue
I would pay close attention to where the screen and digitiser plugs in ,I would also give the connectors (both screen and board) a clean with some isopropyl alcohol and some cotton buds/Qtips as iv had screens act strangely and it was microscopic dirt I couldn’t see on the connectors
Hopefully this helps
Any questions please ask
Thanks:-)