Flutter Firebase Messaging not receiving messages from backend on iOS (works on Android and with Firebase Console)

Question

I'm having an issue with Firebase Cloud Messaging in my Flutter app where:

• On iOS: Messages only work when sent via Firebase Console, but not when sent from my backend
• On Android: Messages work perfectly from both Firebase Console and backend
• I'm using the firebase_messaging package 15.2.5 and firebase core 3.13.0

Key Additional Information:

• My app subscribes users to a topic (using their user ID) when they login
• The backend sends notifications to these user-specific topics
• Topic subscription appears to work (no errors), but messages never arrive on iOS

What I've Tried:

. Verified APNs certificates are properly configured in Firebase Console . Ensured the payload structure matches what iOS expects . Confirmed the device token is being correctly registered . Checked that background modes are enabled in Xcode . Verified notification permissions are granted

Topic subscription in Flutter:

// Called during user login
subscribeToNotification(String userId) async {
 try {
   await FirebaseMessaging.instance.subscribeToTopic(userId);
 } catch (e) {
   print('Subscription error ${e.toString()}');
 }
}

Backend sending code (TypeScript):

const sendDataNotif = async (token: string, data: object, topic: string) => {
  try {
    const headers = {
      Authorization: 'Bearer ' + token,
    };
    const payload = {
      message: {
        topic,
        data,
      },
    };
 
    return await axios({
      method: 'post',
      url: `https://fcm.googleapis.com/v1/projects/${key.project_id}/messages:send`,
      data: payload,
      headers,
    });
  } catch (error: any) {
    console.log(error.message);
  }
};