Left bumper does not work only when left thumbstick pushed forwards

So technically it's a scuf infinity 4ps, not a Sony branded ds4, but after taking it apart it looks near identical to the regular Sony controller, aside from an extra board for the paddles on the back and some extra ribbon cables to power said paddles. I've taken apart and rebuilt ds4's a few times and I've never come across something like this. The left bumper does not work while the thumbstick is being pushed forward. For example, the left bumper is responsible for the run function in hitman 3, if I am moving forward (away from the camera) the left bumper does nothing and I cannot run. If I am moving backwards or to either side (towards and to the left and right of the camera) the left bumper works and I am able to run. If I am successfully running in any of the three working directions, and then try to move forwards, the action immediately stops and the bumper ceases to function. Additionally, the bumper works when navigating menus with no problems. I've taken it apart and cleaned it to no avail and now I'm completely at a loss. I've never heard of one button or switch 'interfering' with the function of another. Any ideas?