虽然我的Braintree交易成功，但在Codeigniter中没有重定向到成功页面

I'm struggling with a piece of code for more than a week. I implemented Braintree, using the PHP SDK, in my Codeigniter app and I have this issue:

1. The user selects the product he wants, enters the shipping method then the Credit Card info;
2. The Credit Card is processed on the Braintree servers and returns a success or error flag;
3. If successful the order is added in the Database and a confirmation email is sent both to the merchant and the customer
4. Finally, the user is redirected to the success page or the error page if the transaction failed.

Everything works as expected in sandbox mode but when I go into production mode, the redirect fails, the page is redirected back to the order confirmation, even though the CC was charged and both emails sent.

Here is my controller:

function order_confirmation()
{
    require_once('application/libraries/braintree/lib/Braintree.php');

    Braintree_Configuration::environment('production');
    Braintree_Configuration::merchantId('MY_MERCHANT_ID');
    Braintree_Configuration::publicKey('MY_PUBLIC_KEY');
    Braintree_Configuration::privateKey('MY_PRIVATE_KEY');

    if ($this->input->post('checkout'))
    {   
        $price = 24.99;
        $quantity = 1;  

        if ($this->input->post('shipping') == 1)
            $shipping_price = 0;
        elseif ($this->input->post('shipping') == 2)
            $shipping_price = 6.99;

        $amount = $price * $quantity + $shipping_price;

        //BrainTree payment process
        $result = Braintree_Transaction::sale(array(
            'amount' => $amount,
            'creditCard' => array(
                'number' => $this->input->post('credit_card_number'),
                'expirationDate' => $this->input->post('expiration_month') . '/' . $this->input->post('expiration_year'),
                'cvv' => $this->input->post('cvv')
            ),          
            'options' => [
                'submitForSettlement' => True
            ]
        ));

        if ($result->success)
        {   
            // I left only the first and last name field to save up space
            $first_name = $this->db->escape($this->session->userdata('first_name'));
            $last_name = $this->db->escape($this->session->userdata('last_name'));                          
            $date_created = $this->db->escape(time());

            // Add the order
            $this->shop_model->add_order($first_name, $last_name, $transaction_id, $date_created);

            $order_id = $this->db->insert_id();

            $product_id = $this->db->escape($this->input->post('product_id'));

            // Add the order items
            $this->shop_model->add_order_items($order_id, $product_id, $quantity, $price);          

            $data['site_name'] = $this->config->item('website_name');
            $data['order'] = $this->shop_model->get_order($order_id);
            $data['order_items'] = $this->shop_model->get_order_items($order_id);

            $customer_email = $this->session->userdata('email');

            // Send the email notification to the merchant
            send_html_email('order_confirmation_merchant', $this->config->item('primary_email'), $this->config->item('website_name'), $this->config->item('primary_email'), 'shop', $data);

            // Send the order confirmation to the customer
            send_html_email('order_confirmation_customer', $this->config->item('primary_email'), $this->config->item('website_name'), $customer_email,  'shop', $data);     

            redirect(SHOP . '/checkout-success');
            // header("Location: checkout-success");                            
            // echo '<script language="JavaScript">document.location.href="' . base_url() . SHOP . '/checkout-success' . '"</script>' . "
";
        }
        else
        {           
            redirect(SHOP . '/checkout-error');
        }
    }

    $this->template->set_template('no_sidebar');

    $this->template->write('meta_description', 'Order confirmation');
    $this->template->write('meta_keywords', 'Order confirmation');
    $this->template->write('title', 'Order confirmation');
    $this->template->write_view('header', 'frontend/header');
    $this->template->write_view('section', 'frontend/shop/order_confirmation'/*, $data*/);
    $this->template->write_view('footer', 'frontend/footer');
    $this->template->render();  
}

As you can see I tried serveral redirect methods (Codeigniter redirect(), native PHP, Javascript) with no success. As mentioned before, I do see the success page in sandbox mode and the error page if I enter a bogus CC, but no success page, only my order confirmation form, in production mode, even though the CC is charged, the order added in the database and the emails sent.

I would like to mention that the website has a SSL certificate and I tested using Mozilla Firefox, Google Chrome, Microsoft Edge. There are no JS errors, no PHP warnings.

Any answer would be greatly appreciated. Thank you!