Agent Scripting Data Action returning 400 Bad Request on WhatsApp trigger

I’m completely stumped as to why the ServiceNow Data Action fails with a 400 Bad Request when invoked via Architect on WhatsApp inbound. The payload structure matches the SNOW REST API schema exactly. The issue only occurs when channel_type is whatsapp. Here is the payload being sent:

{
 "sys_id": "12345",
 "channel": "whatsapp",
 "contact_id": "98765"
}

The same flow works for voice. Is there a specific header requirement for WhatsApp webhooks?