Are you syncing both the Administration.Account and System.User entities to the device? The documentation about an offline-first approach states that it is not possible to use more than one entity from a generalization or specialization relation. If you sync both the generalization and specialization, it is unsure which table will be available on the device and under what name. This looks like the issue you're experiencing.
In terms of retrieving a list of System.User objects in a nanoflow on a device, be mindful of the fact that entity access is taken into account. Since the entity access rule for System.User objects is always [id = ‘[%CurrentUser%]’], you will never be able to sync and retrieve other System.User objects apart from your own.
Have you thought about maintaining the desired attributes in separate entity? You would then be able to apply the necessary entity access rules and sync this entity to the device, circumventing any issues with inheritance.