When using getUpdatesForDevice() in the Android Support SDK (5.1.2) the updates are incorrect until you open the tickets list (or the ticket itself). So for example, if you receive a new ticket response while the app is closed, and then use getUpdatesForDevice() on launch (after the SDK is initialised) then it will incorrectly report that there are no updates. Opening the tickets list fixes it. Using refreshRequest() on the ticket ID will also fix it which can be called when push notifications are received, but this does not work when the app is closed.
It might be possible to query all the user's ticket IDs on launch and call refreshRequest() on each one before using getUpdatesForDevice() but this should be the SDK's job - there is no documentation around needing to do this.
Have you been able to replicate this issue on other versions or just 5.1.2? Are these updates being received within the last hour of the previous time the getUpdatesForDevice method was used? If so, keep in mind that the response will be cached for one hour.