Oy, having the same problem with a 2015 13” MacBook Retina. To fix the issue after consulting this site, I got a used replacement display assembly. $275 later… The problem persists. It is less bad, but different horizontal and vertical lines appear on the screen. Despite restarts, PRAM resets, attempts to re-seat the cable, the works. And they don’t show up on a 2nd monitor.
Don’t know if this was the case with the 1st display assembly (which grew worse and worse, eventually developing solid black blotches of nothingness randomly across the screen), but the replacement display appears to have a “soft spot” on the screen, at the nexus of the horizontal and vertical lines — a place where, if you touch it or put pressure on the screen, the colors of the lines change, but don’t go away. And if you press hard enough, black blotches appear. They are temporary, and disappear after a while. For now…
Like I said, “oy.”