Pro Controller L, R ZL, ZR Buttons Not Responding

Hi there, first time posting. I have a pro controller that has spontaneously lost all response from the ZL & ZR buttons. There seems to be continuity from the contact pads on the ribbon cable down to at least the first vias on the button board, and i’ve checked continuity on the button board’s ribbon cable that runs to the main board. I’ve also replaced both those ribbons cables anyway, just in case. My issue now is that I don’t have a schematic or diagram or anything to see if i can hunt down a bad solder joint or broken trace. Every other input (analog, face buttons) seems to be working fine, so i’m hoping the problem isn’t on the main board, but either way I’m kind of lost at the moment. Any ideas on how to proceed?