Yeah I tried both — turning the keyboard on with the pedal plugged will not generate any sustain pressed or not press, while turning the keyboard on without the pedal plugged, then plugging it after, will always generate sustain (pressed or not pressed)
So I'm not sure if its about the polarity that is the issue. My SP-3 pedal works fine with it.
The Korg DS-1H pedal has 3 (TRS), my SP-3 only has 2 (TS).