L2 Trigger influences Right joystick

I've been trying to repair a Dualshock 4 controller (JDM-055) that had the following issue:

Pressing the L2 trigger induced an input on the right joystick's Y axis. So pressing L2 made the camera go up.

What I've attempted: Opening up the controller, cleaning it, and making sure all the contacts were clean and aligned. Once that was done, I test it again on gpadtester and it was still displaying the same fault. So I thought maybe the joystick was faulty and leaking current somehow and de-soldered it. It's still behaving the same even without any joystick there, it just pegs the value further up. Is this a known issue? Is the main PCB toast? I can't think of anything else other than a drifted resistor somewhere or a PCB crack, which probably wouldn't be worth repairing. Any tips appreciated!

Image of what gpadtester looks like when pressing L2 with no joystick installed. When released, the input hovers near the edge of the circle. When pressing, it goes up.

Hi again, I've changed the circuit membrane, and now the two triggers are stuck on 1 no matter whether I press them or not. I've checked and both the membrane and the gaskets are seated properly. All other buttons are working. The right joystick is still empty. Any idea what the issue could be? Thanks!