AlarmStatusHistory returns 0 results when requesting application/json

If you send a GET request for /ws/AlarmStatusHistory with the Accept header set to ‘application/json’, you get a response, but there are zero items in the response message.

If the same request is made with Accept header as ‘text/xml’ or no Accept header, the response contains items as expected.

Is this a bug, or expected behaviour?

This sounds like a bug. I recommend reporting this to Digi Tech. Support.