Preserve the array keys while sending data in json format to custom api endpoint magento2

I hace created a custom api endpoint to receive data from a third party service. These are my modules files

etc/webapi.xml

<?xml version="1.0" ?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">
    <route url="/V1/matrid-nextapi/orders" method="POST">
        <service class="Matrid\NextApi\Api\OrdersManagementInterface" method="postOrders"/>
        <resources>
            <resource ref='anonymous'/>
        </resources>
    </route>
</routes>

etc/di.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Matrid\NextApi\Api\OrdersManagementInterface" type="Matrid\NextApi\Model\OrdersManagement"/>
</config>

Api/OrdersManagementInterface.php <?php /** * Copyright © All rights reserved. * See COPYING.txt for license details. */ namespace Matrid\NextApi\Api;

interface OrdersManagementInterface
{

    /**
     * 
     * @param mixed $Orders
     * @return mixed[]
     */
    public function postOrders($Orders);
}

This is Model file for interface.

Model/OrdersManagement.php

<?php
/**
 * Copyright ©  All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Matrid\NextApi\Model;

use Matrid\NextApi\Model\OrdersFactory;

class OrdersManagement implements \Matrid\NextApi\Api\OrdersManagementInterface
{
    /**
    *
    * @param mixed $Orders
    * @return mixed[]
    */
    public function postOrders($Orders)
    {
        $response = $Orders;
        $returnArray = json_encode($response);
        return $returnArray; 
    }
}

Now I have created a custom file to test the API.

custom-test.php

<?php
$data = array (
  'Orders' => 
    array (
      0 => 
        array (
          'ID' => 19558599,
          'Brand' => 'Brand',
          'Destination' => 'NextRDC',
          'DateTimeStamp' => '2021-01-05T20:37:29+00:00',
          'Currency' => 'GBP',
          'Items' => 
             array (
               0 => 
                 array (
                   'ItemID' => 12345678,
                   'EAN' => '1234567890123',
                   'BrandSKU' => '12345678',
                   'Quantity' => 1,
                   'PromiseDate' => '2021-01-09',
                   'Price' => 67,
                 ),
             ),
         ),
     ),
  )
;

$curl = curl_init("https://staging.nonnon.co.uk/index.php/rest/V1/matrid-nextapi/orders");
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($data));
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HTTPHEADER, array("Content-Type: application/json"));

$result = curl_exec($curl);

$data = json_decode($result, true);

var_dump($data);
?>

The output that I am receiving is this

[{"ID":19558599,"Brand":"Brand","Destination":"NextRDC","DateTimeStamp":"2021-01-05T20:37:29+00:00","Currency":"GBP","Items":[{"ItemID":12345678,"EAN":"1234567890123","BrandSKU":"12345678","Quantity":1,"PromiseDate":"2021-01-09","Price":67}]}]

If you check the output,the $data variable that is passed as post data in custom-test.php has lost its "Orders" key. And I know this has happened because of passing $Orders as comment in OrdersManagement.php file.

Now my question is what shall I do in order to prserve the Orders key. What should be the proper way to do this. Whatever json data is passed I shall be able to receive and print the entire data. Could someone plz guide me a way to do this.