Firebase v1 HTTP api does not support nested JSON objects

CURRENT SITUATION

We send a POST request Firebase legacy URL https://fcm.googleapis.com/fcm/send to deliver notification JSON payload to our Android application. Our notification payload is a nested JSON object within a standard data. Our sample payload from Postman is given below

{
 "registration_ids": [
   "${registration_id}"],
   "priority":"HIGH", 
 "data": {
   "notification": {
     "title": "Notification Data Title From Postman",
     "body": "Notification Data Body From Postman",
     "imageurl": "https://d2x51gyc4ptf2q.cloudfront.net/content/uploads/2015/10/GettyImages-51205958-700x367.jpg"
   },
   "fcm_options": {
           "analytics_label": "postman"
     }
 }
}

The request headers are as follows:

Content-Type: application/json; charset=utf-8`
Authorization: key={{server_key}}

In this example, the nested JSON object within the data object as shown below is used to successfully construct the notification.

"data": {
       "notification": {
         "title": "Notification Data Title From Postman",
         "body": "Notification Data Body From Postman",
         "imageurl": "https://d2x51gyc4ptf2q.cloudfront.net/content/uploads/2015/10/GettyImages-51205958-700x367.jpg"
       }
}

The server key is obtained from the Firebase console's project settings. This payload is successfully delivered to an Android app on an appropriate device to create a notification.

PROBLEM

We want to migrate to Firebase v1 HTTP request APIs. I am attempting to use OAuth2 Tokens to authenticate and deliver notifications to the Android device. The payload that I want to send through V1 api is given below

{
  "message": {
    "token": "${token}",
    "data": {
      "notification": {
        "title": "Notification Data Title From v1 API",
        "body": "Notification Data Body From v1 API",
        "imageurl": "https://d2x51gyc4ptf2q.cloudfront.net/content/uploads/2015/10/GettyImages-51205958-700x367.jpg",
        "extra_key": "extra_value"
      }
    },
    "android": {
      "priority": "HIGH",
      "fcm_options": {
        "analytics_label": "Test_Command_line"
      }
    }
  }
}

When we make a HTTP POST request to the v1 HTTP API, I get a 400 error with the following error message.

{
  "error": {
    "code": 400,
    "message": "Invalid JSON payload received. Unknown name \"title\" at 'message.data[3].value': Cannot find field.\nInvalid JSON payload received. Unknown name \"body\" at 'message.data[3].value': Cannot find field.\nInvalid JSON payload received. Unknown name \"imageurl\" at 'message.data[3].value': Cannot find field.\nInvalid JSON payload received. Unknown name \"extra_key\" at 'message.data[3].value': Cannot find field.",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.BadRequest",
        "fieldViolations": [
          {
            "field": "message.data[3].value",
            "description": "Invalid JSON payload received. Unknown name \"title\" at 'message.data[3].value': Cannot find field."
          },
          {
            "field": "message.data[3].value",
            "description": "Invalid JSON payload received. Unknown name \"body\" at 'message.data[3].value': Cannot find field."
          },
          {
            "field": "message.data[3].value",
            "description": "Invalid JSON payload received. Unknown name \"imageurl\" at 'message.data[3].value': Cannot find field."
          },
          {
            "field": "message.data[3].value",
            "description": "Invalid JSON payload received. Unknown name \"extra_key\" at 'message.data[3].value': Cannot find field."
          }
        ]
      }
    ]
  }
}

But when the data payload is not nested as given below

"data": {
    "title": "Title that is not nested",
    "body": "Body that is not nested",
    "imageurl": "https://timedotcom.files.wordpress.com/2016/08/margot-robbie-beer-shower.jpg",
}

then I get a 200 OK response. The response body is given below

{
  "name": "projects/<project_id>/messages/<message_id>"
}

DESIRED BEHAVIOUR

I want to be able to deliver nested data payloads using Firebase v1 HTTP API to our Android app so that the notifications can be rendered. How can I achieve that given that I am getting errors when I send nested payloads? This is the program that I have been using to make HTTP requests. The program uses Google-API clients, FreeMarker, Firebase-Admin, and OKHttp.

public class FCMMessaging {

    public static void main(String[] args) throws IOException, FirebaseMessagingException {

        // File was downloaded from Firebase console URL: https://console.firebase.google.com/project/<project-id>/settings/serviceaccounts/adminsdk
        try(InputStream is =
                FCMMessaging.class.getClassLoader().getResourceAsStream("firebase-admin-service-account.json");) {

            GoogleCredential googleCredential = GoogleCredential
                    .fromStream(is)
                    .createScoped(Collections.singletonList("https://www.googleapis.com/auth/firebase.messaging"));
            googleCredential.refreshToken();
            String bearerToken = googleCredential.getAccessToken();

            Configuration cfg = new Configuration();

            // Where do we load the notification payload templates from:
            cfg.setClassForTemplateLoading(FCMMessaging.class, "");

            // Some other recommended settings:
            cfg.setDefaultEncoding("UTF-8");
            cfg.setLocale(Locale.US);
            cfg.setTemplateExceptionHandler(TemplateExceptionHandler.DEBUG_HANDLER);
            Map<String, Object> templateMap = new HashMap<>();
            templateMap.put("token",
                    "my-token-id");

            Template template = cfg.getTemplate("notification_payload.json");
            StringWriter stringWriter = new StringWriter();
            template.process(templateMap, stringWriter);
            RequestBody requestBody = RequestBody.create(stringWriter.toString(), MediaType.get("application/json; charset=utf-8"));
            Request request = new Request.Builder().url("https://fcm.googleapis.com/v1/projects/<project-id>/messages:send")
                    .addHeader("Authorization", "Bearer " + bearerToken).post(requestBody).build();

            OkHttpClient client = new OkHttpClient();
            try (Response response = client.newCall(request).execute()) {
                System.out.println(response.body().string());
            }
        } catch (TemplateException te) {
            te.printStackTrace();
        }
    }
}