Well…if the external is working correctly then the internal display has the issue either within the lid or the TCON board which is held in the upper case area or the ribbon cables between the two.
Review this guide [guide|123893] on what’s required to get to the area in question. Carefully inspect of wear on the ribbon cables as well as damage with the Chin plate area just below the screen as the chin plate glass could have punctured the ribbon cables.
As you can see a replacement display assembly is not cheap!