Whenever I press the touchpad toggle key on my keyboard, I see the notification for touchpad off for a microsecond before it again switches to touchpad On. A custom shortcut for it works fine.
The terminal returns these hardware codes when I run sudo showkey --scancodes and press the touchpad toggle key:
0x1d 0xe0 0x5b 0x6f 0xef 0xe0 0xdb 0x9d