Maybe sharing some screenshots of what you’re seeing would help?
As mentioned above, this labs endpoint queries all of the devices in the SmartThings API for their reported health status and checks for their offline status. If the status is exactly 'OFFLINE', then its added to the list of offline devices.
You could query the status of that individual device directly as shown in the following thread to see what the SmartThings API is reporting for it. It’s a bit more of a pain to test as you can’t do it in your browser and instead have to put some headers together.