In our app we use Firebase Cloud Messaging to send push notifications to our users. We save the registration tokens of our users in our SQL database and when a user logs in or out we use the Firebase API to manage his topic subscriptions.
This works out pretty well in most of the cases but now the topic subscriptions return the following error for some of our registration tokens:
The provided registration token is not registered. A previously valid registration token can be unregistered for a variety of reasons. See the error documentation for more details. Remove this registration token and stop using it to send messages
In which cases does Firebase delete previously registered registration tokens? I can not find any specific information about this in the error documentation.
Thanks for your help!