Hi
The cables for the display are pretty tricky to get to on this model so it can be quite hard to see if there is any debris in the connector so what I would recommend is removing the battery (just so it’s no longer adhered to the chassis but still soldered to the logicboard,removing the battery helps get more room to access the back of the logic board)and clean the connectors(both screen and board) with isopropyl alcohol
If that doesn’t work try the old screen and see if that shows similar results
If it shows similar results the connector on the board may be damaged and will need replacing by someone who has good micro soldering skills
Hopefully this helps
Any questions please ask
Thanks:-)