ServiceNow Table API 400 Bad Request on SMS Conversation Start

Status 400 Bad Request returned by ServiceNow Table API via Genesys Cloud Data Action when triggered by SMS conversation start. Payload validated against SNOW docs; fields map correctly. Tested via Postman with identical JSON structure-successful creation. Verified webhook signature and OAuth token validity. Architect flow uses ‘Create ServiceNow Record’ action with static mapping. No logs in ServiceNow indicating schema mismatch. Is there a specific header requirement for digital channel triggers that differs from manual webhook invocations?