Set Participant Data action not persisting custom vars across IVR nodes

I’m trying to pass custom variables through an IVR flow using the Set Participant Data action. The JSON payload looks correct:

{ "customVar1": "value1" }

But when I hit the next node, the variable is null. I’ve checked the flow logs and the action reports success. Is there a specific scope issue I’m missing? We’re on the latest patch version.