Session Handover Failures in CXone Studio when Integrating NICE Cognigy Voicebots

We are seeing session handovers fail when moving from a Cognigy voicebot back to CXone Studio. The GetRESTProxy call returns a 200, but the session drops before the agent prompt. The JSON payload looks correct:

{"transferType": "session", "targetId": "studio_script_id"}

Is there a specific header required for the handover endpoint, or is this a known issue with the latest Studio update?