iOS devices never receive data-only message

[Pedrozena]

Describe your environment

• Operating System version: Amazon Linux 2 (AWS Lambda)
• Firebase SDK version: 6.5.0
• Firebase Product: messaging
• Python version: 3.10
• Pip version: _____

Describe the problem

I'm having the same problem reported here https://stackoverflow.com/questions/69244666/flutter-with-firebase-messaging-ios-device-doesnt-receive-data-only-messages
Since the question in unanswered for 3 years, I guess it's fine to open an issue here.

Android devices receive data-only messages, while iOS ones receive regular messages with non-null notifications.
While using legacy FCM api with the following payload:

{
  "to": token,
  "priority": "high",
  "content_available": true,
  "data": {
    "new_message": {
      "sender_uid": sender_uid,
      "message_payload": message,
    },
  },
}

iOS devices regularly receive also data-only messages.

Steps to reproduce:

I'm sending messages to my devices with the following code:

from firebase_admin import messaging

def send_message_to_token(token, notification=None):
    data = {"foo": "bar"}
    apns = None
    android = None
    if notification is None:
        apns = messaging.APNSConfig(
            payload=messaging.APNSPayload(
                aps=messaging.Aps(content_available=True, custom_data={"gcm.message_id": str(uuid.uuid4())})
            )
        )
        android = messaging.AndroidConfig(priority="high")
    else:
        notification = messaging.Notification(**notification)
    message = messaging.Message(data, notification=notification, token=token, apns=apns, android=android)
    messaging.send(message, app=get_app())