Ive honestly never used the l + r buttons in gameplay for SNES but I’m pretty sure they are functioning. Its possible for your controller they may be buttons 6 and 7 rather than 4 and 5 but idk. This is my official gamepad configuration file that was created by retroarch (I didn’t know what I was doing when I created the first file above):
https://github.com/libretro/retroarch-joypad-autoconfig/blob/master/udev/USBgamepad.cfg