Data Action 400 Bad Request on Custom Object Update Payload

NICE CXone API & Integrations
1

Anyone know why the Platform API rejects this specific payload for a custom object update via a Genesys Cloud Data Action? The integration uses a Premium App with multi-org OAuth, and the endpoint is /api/v2/custom-objects/instances. We are receiving a 400 Bad Request with message Invalid JSON input. The payload structure matches the schema definition exactly.

{
 "externalId": "EXT-12345",
 "fields": {
 "status": "active",
 "priority": 1
 }
}

Any insights on strict validation rules?