iOS push notifications failing after several days

I’m facing an issue where a new iOS user will initially be able to receive FCM push notifications, but after a few days, stops being able to receive them. However, the server which is sending the push notifications still receives a success message from FCM:

{ multicast_id: 5364950433438816000, 
  success: 1, 
  failure: 0, 
  canonical_ids: 0, 
  results: [ { message_id: '0:1471123826177313%13c7f1c613c7f1c6' } ] } 

After I tried manually sending a notification to the relevant FCM token through the firebase console (which failed), the server then starts getting a failed message from FCM:

{ multicast_id: 4711697033517732000, 
  success: 0, 
  failure: 1, 
  canonical_ids: 0, 
  results: [ { error: 'NotRegistered' } ] } 

This led me to believe that the FCM token my server had was somehow out of date; however, I double checked, and the FCM token my server is using is consistent with the token given by calling FCM.on('refreshToken', (token) => ... in the app.

Is there some other reason why this might be occurring?

[evollu]

do you get refreshToken called all the time?

[wsun]

In case this is helpful for others - I experienced a similar issue, where the FCM token had not changed, but at some point the user was no longer able to receive remote push notifications. This was because we failed to regularly keep the APNs device token up to date: we did not call registerForRemoteNotifications appropriately on every app launch (i.e. in AppDelegate), as Apple requires. This method then triggers the Firebase-swizzled didRegisterForRemoteNotificationsWithDeviceToken to map the new APNs device token to the current FCM token.

This issue only seemed to occur on devices where we were regularly installing new app builds, so I suspect the APNs device token changes under certain circumstances, even when the FCM token is still the same.