发送到设备组令牌时，Firebase Push通知仅显示“来自”

I'm developing a service for handling push notifications that integrates with Firebase cloud messaging. When I send a new message and specify a device token, it works fine. I get the notification title, body etc. However, when I specifiy a device group ID (which I get as a response when I register a new group with the device token), the device gets a notification, but it's blank - only showing the "from" field which is the sender ID. None of the notification title, body etc. show.

Code for message construction

func FcmNewMessage(title, body, collapseKey, token string) *messaging.Message {
    oneHour := time.Duration(1) * time.Hour
    message := &messaging.Message{
        Notification: &messaging.Notification{
            Title: title,
            Body:  body,
        },
        Android: &messaging.AndroidConfig{
            TTL:         &oneHour,
            CollapseKey: collapseKey,
        },
        APNS: &messaging.APNSConfig{
            Headers: map[string]string{
                "apns-collapse-id": collapseKey,
            },
        },
        Webpush: &messaging.WebpushConfig{
            Notification: &messaging.WebpushNotification{
                Icon: "https://my-server/icon.png",
            },
            Headers: map[string]string{
                "urgency": "high",
                "topic":   collapseKey,
            },
        },
        Token: token,
    }
    return message
}

As you can see above - "token" argument can either be a device token or a device group notification key token. It's when the device group token is used that there is issues.

Below are the client notifications:

When using the device token:

{
  "from": "53xxxxxxxxxx",
  "priority": "high",
  "notification": {
    "title": "test",
    "body": "test",
    "icon": "https://my-server/icon.png"
  },
  "collapse_key": "collapseMe"
}

When using device group token:

{
  "from": "53xxxxxxxxxx"
}

I should also mention that when a device group token is used, the response I get from FCM is:

projects/projectid-123/messages/

So there is no message ID at the end. Normally the response would be:

projects/projectid-123/messages/<messageId>