typically, a device class is handled as an autoobject in order to have one global instance of the device class. This is very convenient, because autoobjects can be accessed from everywhere in your project.
This is also the case in your example and Application::DeviceClass is accessed as autoobject.
Please consider the livetime of autoobjects. Everytime, the autoobject is no more in use, the instance will be deleted by the Garbage Collector. When the timer expires, a new instance of the device class will be created. Within the Prototyper on PC, the Garabage Collector is running after a couple of seconds - this explains, why you get sometimes the expected value and why the content is lost after some seconds.
In order to prevent the Garbage Collector from freeing an autoobject, please add a variable to the root class, set the type according to your device class (in your case Application::DeviceClass) and the default value to the instance (autoobject) of the class (in your case Application::Device).
Does it work?