I figured out what it is...
First, make sure you have the switch on the back set to KBD. (It moved on me once and I spent a while looking at the front panel for nothing).
Second, make sure the mod wheel switch is set to LFO amount.
That will prevent the modulation from happening on its own until you move the wheel. You can play the keyboard normally and add trem to it as you'd expect. However, it will still trigger the tremolo if you push it forward on its own, even without playing the keys. A strange implementation, but it has a workaround.