I had the same problem. I finally opened up my controller and found small bits on paper inside. Turns out that Microsoft puts stickers over a screw on the motherboard so that they know if you've tampered with the controller. For me, that sticker had fallen off and bits of it were stuck in different buttons including my left bumper. Try opening your controller and just vacuuming the insides of the controller, or use tweezers to remove anything blocking the button