2020-12-11 18:39:36.965 14746-14746/your.bundle E/RNFirebaseMsgReceiver: Background messages only work if the message priority is set to 'high'

This is something you can find in your adb logcat output when sending a push notification (cloud message) and your app is in background.

More specifically, the problem is mostly related to the idle status of your device.

This is explained in a dedicated Firebase doc, particularly in the “Using FCM to interact with your app while the device is idle” section.

If you are sending a message with curl, Postman and so on, you can add something to your JSON in order to set priority.

So a JSON body like this:

{ 
  "to": "token_here", 
  "notification": { 
    "title": "hey from Lorenzo", 
    "body": "great article dude!" 
  }, 
  "data": { 
    "body": "normal priority notification", 
    "title": "here we are", 
  }
}

becomes:

{ 
  "to": "token_here", 
  "android": {
        "priority": "high"
  },
  "priority": 10,
  "notification": { 
    "title": "hey from Lorenzo", 
    "body": "great article dude!" 
  }, 
  "data": { 
    "body": "normal priority notification", 
    "title": "here we are", 
  }
}

When performing your POST requesto to https://fcm.googleapis.com/fcm/send please remember to add a Content-type: application/json header and a Authorization header with value key=<server_key> that you can retrieve from the “Cloud Messaging” section of your project settings in the Firebase console.