Why does this setting block outbound message delivery via Platform API?

Why does this config cause immediate failures when triggering outbound messages through the /api/v2/conversations/messages endpoint? Our Premium App integration handles high-volume notifications.

The request returns a 400 Bad Request with error code ‘invalid_message_content’. This occurs despite valid JSON payloads and correct OAuth scopes for the service account.

We are using the latest Node.js SDK version 2.1.4. The issue persists across multiple organizations, suggesting a potential platform-side validation change we are missing.