Set Participant Data action failing with 400 in Java SDK orchestration

Struggling to figure out why the Set Participant Data action fails with a 400 error.

My Java service orchestrates IVR flows via the API. I construct the payload with custom variables like user_id and session_token.

The JSON structure matches the schema perfectly. Yet, the flow execution stops immediately upon hitting this specific data action block.

Is there a strict type enforcement I am missing in the participantData map definition?