Straight Shadow on the left
Not visible in white/light screen
iPhone 6, iOS 12
Not visible in white/light screen
iPhone 6, iOS 12
This would appear to be a minor variation on an inverter setting. If present when you attach an external display it is something to do with the graphics chip. If only present on the internal display, then the inverter cable or chip may be having problems. Bring it in to Apple either way, as this issue is not user repairable.
Straight Shadow on the left