I’d start by crossing cables to different displays. Then by actually switching cables. This way you can narrow it to one of three things. Before the cable, the cable, or after the cable. In my experience it tends to be before the cable if it’s an entire line on a display. You can use the schematics to narrow it down to a certain pin which should be fairly easy, or you can check each one. But I’d bet the display board itself has a bad solder joint.

Edit: just reread your post and saw it uses one ribbon cable. Still, Same thoughts since it’s a whole line, but my trouble shooting technique is useless.