Firestore write fails in iOS native despite valid Flutter login

I'm building a Flutter app that uses Firebase Auth and Firestore. Everything works fine on the Flutter side.

Now, I need to save battery info from iOS native Swift code (in AppDelegate.swift) so it can run in the background — which Flutter doesn't support directly.

To do this, I'm using a secondary FirebaseApp instance in Swift (named "flutter-app"), and I'm trying to write to Firestore using the existing user who was already authenticated in Flutter.

However, when I try to write to Firestore from Swift, I get this error:

[FirebaseFirestore][I-FST000001] WriteStream (313536646433373138) Stream error: 'Permission denied: Missing or insufficient permissions.'
[FirebaseFirestore][I-FST000001] Write at users/{uid}/devices/{deviceId} failed: Missing or insufficient permissions.

Even though:

• auth.currentUser in Swift is not nil

• The uid matches the one from the Flutter side

• Firestore rules are correctly configured to allow users to write to their own documents

• The same code path (writing to users/{uid}/devices/{deviceId}) works fine in Flutter

• I initialized the secondary FirebaseApp using the same GoogleService-Info.plist as in Flutter.

• I verified that FirebaseApp only initializes once and reuses the instance afterward.

• I used Auth.auth(app: app) and checked that currentUser is valid.

• I expected the write to Firestore to succeed, just like in Flutter, since the user is already logged in and Firestore rules allow it.

Here’s the relevant Swift code:

if FirebaseApp.app(name: "flutter-app") == nil {
    let path = Bundle.main.path(forResource: "GoogleService-Info", ofType: "plist")!
    let options = FirebaseOptions(contentsOfFile: path)!
    FirebaseApp.configure(name: "flutter-app", options: options)
}

let app = FirebaseApp.app(name: "flutter-app")!
let db = Firestore.firestore(app: app)
let auth = Auth.auth(app: app)

guard let currentUser = auth.currentUser else {
    print("User not logged in")
    return
}

// ... build docRef and call setData() ...

Here are the Firestore rules:

rules_version = '2';

service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read, write, update: if request.auth != null && request.auth.uid == userId;

      match /devices/{deviceId} {
        allow read, write, update: if request.auth != null && request.auth.uid == userId;
      }
    }
  }
}

If there's anything I'm missing about how authentication works across native/Flutter with secondary FirebaseApp, I’d really appreciate any help or suggestions!