There is a time limit to how long the app will register for a push if it hasn't been used for a while (a week, you can set this for longer if you install your own SIPIS). This is to prevent SIPIS from registering the account indefinitely if the app is deleted without resetting it first.
We do have a way of renewing this silently. But if a push is not for a call, it will not always be able to wake the app if it is not in the device's memory. And if they haven't used the app in a week, iOS has almost certainly removed the app from device memory even if it is still in recent apps. So the automatic renewal will often fail on iOS. It is more reliable on android, but even there, there may be times when it fails (e.g. temporary network issue, the device is off...). In case the silent renewal fails, you will get the expires message. To renew, simply open the app and wait for the account to register. Push will be renewed every time you use the app.
Note: If there is some issue registering the account for push, it might expire much sooner.