Firebase Push Notifications not showing on iOS

What I Have:

1. A Python script which sends data-only notification messages to my device/emulator using firebase_admin.messaging.send
2. A Flutter mobile app which listens for the incoming messages (FirebaseMessaging.onMessage.listen and FirebaseMessaging.onBackgroundMessage)
3. The data message is parsed and a push notification is shown using flutter_local_notifications

The issue:

The notifications don't work on just iOS. Specifically, the firebase message handlers (FirebaseMessaging.onMessage.listen and FirebaseMessaging.onBackgroundMessage) don't fire for iOS when I send a notification message from my backend.

What I Tried / Things That Do Work:

1. On Android, the entire process works
2. APNs token prints successfully from Flutter
3. On my iOS device, sending a test notification from Firebase Admin Console works, so I'm guessing APNs is setup correctly
4. On my iOS device, local push notifications work when I trigger one from main() at startup (manually using FlutterLocalNotificationsPlugin.show)
5. On my iOS device, while the Flutter handlers don't fire, didReceiveRemoteNotification in my AppDelegate.swift does fire and prints the userInfo logging statement in XCode

I have no clue how to resolve this or debug further - any ideas?

Code Snippets:

// Info.plist contains
...
    <key>FirebaseAppDelegateProxyEnabled</key>
    <false/>
    <key>FirebaseScreenReportingEnabled</key>
    <true/>
    <key>UIBackgroundModes</key>
    <array>
        <string>remote-notification</string>
        <string>fetch</string>
    </array>
...

//AppDelegate.swift

import Flutter
import UIKit
import Firebase
import FirebaseMessaging
import flutter_local_notifications

@main
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    FlutterLocalNotificationsPlugin.setPluginRegistrantCallback { (registry) in
    GeneratedPluginRegistrant.register(with: registry)}
    FirebaseApp.configure()
    GeneratedPluginRegistrant.register(with: self)

    if #available(iOS 10.0, *) {
      UNUserNotificationCenter.current().delegate = self as? UNUserNotificationCenterDelegate
    }

    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }

  override func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {

   Messaging.messaging().apnsToken = deviceToken
   super.application(application, didRegisterForRemoteNotificationsWithDeviceToken: deviceToken)
  }

  override func application(_ application: UIApplication,
    didReceiveRemoteNotification userInfo: [AnyHashable: Any],
    fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    print("Received remote notification: \(userInfo)") // <--- THIS PRINTS CORRECTLY
    // Forward the notification to Firebase Messaging
    Messaging.messaging().appDidReceiveMessage(userInfo)
    
    // Inform iOS that processing is complete
    completionHandler(.newData)
  }
}

# backend python script notification function

import firebase_admin
from firebase_admin import credentials
from firebase_admin import messaging

def send_push_notification(token, data):
    message = messaging.Message(
        token = token,
        data = data,
        apns=messaging.APNSConfig(
            headers={"apns-priority": "5"},
            payload=messaging.APNSPayload(
                aps=messaging.Aps(
                    content_available=True
                )   
            )   
        )   
    )   

    logger.info('Sending push notification: (%s)' % (token))
    logger.info(data)

    try:
        response = messaging.send(message)
        logger.info('Successfully sent message')
    except Exception as e:
        logger.info(f'Error sending message: {e}')