Background Restricted Apps (Android P or newer)

Starting Jan 2019, FCM will not deliver messages to apps which were put into background restriction by the user (such as via: Setting -> Apps and Notification -> [appname] -> Battery). Once your app is removed from background restriction, new messages to the app will be delivered as before. In order to prevent lost messages and other background restriction impacts, make sure to avoid bad behaviors listed by the Android vitals effort. These behaviors could lead to the Android device recommending to the user that your app be background restricted. Your app can check if it is background restricted using: isBackgroundRestricted().

references:
https://developer.android.com/topic/performance/vitals/
https://firebase.google.com/docs/cloud-messaging/android/topic-messaging