From endrift's last reply, the key clue is that the controller wasn't recognized at all in mGBA — when that happens the keyboard fallback mapping takes over, which is why the bumper seems to act like A. Re-plugging the controller before launching mGBA (or selecting it from the controller dropdown in Settings) usually fixes it. Also worth checking that no other gamepad tool or overlay is grabbing the device first, since that can make mGBA see nothing at all.
Quick way to figure out if it's the controller or mGBA: test the bumper outside the emulator. I made a free browser gamepad tester — https://controller.parkerbrown.photos/ — open it in Chrome/Edge and press your left bumper. If it doesn't light up there, the button itself is dead (hardware). If it does light up, the problem is in mGBA's mapping, so go to Settings > Controllers, clear the L-button binding and re-bind it while pressing the bumper firmly.
Quick way to figure out if it's the controller or mGBA: test the bumper outside the emulator. I made a free browser gamepad tester — https://controller.parkerbrown.photos/ — open it in Chrome/Edge and press your left bumper. If it doesn't light up there, the button itself is dead (hardware). If it does light up, the problem is in mGBA's mapping, so go to Settings > Controllers, clear the L-button binding and re-bind it while pressing the bumper firmly.