Then it is either the video cable or the connections at that cable or on the board.
This is NOT an unknown issue. Louis Rossmann may have a video on these parts and areas. HOWEVER the average owner is just that. Get repair estimates since most don't want to become a computer repair tech.
Thick purple bars when screen is at certain angles, normal at other angles. 2015 12 inch MacBook retina is showing thick purple bars on the screen when it is at certain positions/angles and is showing normal screen at other positions/angles. Rebooting and clearing NVRAM does not change it. The issues seems highly sensitive to the position of the screen, what angle it is at. Everything else is functioning properly(keyboard, mouse, internet..etc) as far as I can tell. Here is a link to a picture and video of the issue.