This issues relates to the "REST APIs" global service. The "Warning Header" attribute has the following definition in RestApis.xml.
Due to the use of Enabled and Disabled in the entry (which should be True and False instead), the attribute is always exposed on the REST SMS as false, then though the value is actually true (see screenshot of UI).
Expected: User seeing an accurate reflection of the server state
Actual: User is mislead as to the real value of the "Warning Header" attribute