How to get custom checkout field values with Woocommerce Checkout Blocks

I added some data with the function :

woocommerce_register_additional_checkout_field(
            array(
                'id'       => 'namespace/newsletter-opt-in',
                'label'    => 'Subscribe to newsletter ?',
                'location' => 'order',
                'type'     => 'checkbox',
            )
        );

I try to check this checkbox but I can’t get the value
However, the data is well returned in my order data column

I tried :

$order = wc_get_order($order_id);
$subscribe = $order->get_meta('namespace/newsletter-opt-in');

But $subscribe is empty