Custom Field created in add_meta_boxes reappearing itself again in the default Custom Metabox

I succeed in creating a metabox with custom field inside, and I restrict it to appear in a custom post type.

//define metabox
function product_info_en() {
    add_meta_box( 'english_info', 'English Info', 'english_product_name_callback', array('product'), 'normal', 'high' );
}

//add to hook
add_action( 'add_meta_boxes', 'product_info_en' );

The code to display it in the product page:

// display in add product admin page
function english_product_name_callback( $post ) {
    //ob_start();
    $content = esc_attr( get_post_meta( get_the_ID(), 'product_desc_en', true ) );

        //here goes the custom field
        echo '<fieldset><div><label><b>English Product Name:</b></label><br/>';
        echo '<input id="product_name_en" type="text" name="product_name_en" style="width:100%; margin:10px 0px"';
        echo ' value="';
        echo esc_attr( get_post_meta( get_the_ID(), 'product_desc_en', true ) );
        echo '"></div></fieldset>';

        //here goes the wp_editor
        echo '<fieldset><div><label><b>English Product Content Info:</b></label><div><br/>';
        echo '<div>';
        wp_editor($content, 'product_desc_en', array(
            'wpautop' => true,
            'media_buttons' => true,
            'textarea_rows' => 10
            )
            );
        echo '</div></fieldset>';
}

Here goes the code that do the saving job:

//save
function enginfo_save_meta_box( $post_id ) {
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
    if ( $parent_id = wp_is_post_revision( $post_id ) ) {
        $post_id = $parent_id;
    }
    $fields = [
        'product_name_en',
    ];
    foreach ( $fields as $field ) {
        if ( array_key_exists( $field, $_POST ) ) {
            update_post_meta( $post_id, $field, sanitize_text_field( $_POST[$field] ) );
        }
     }
    update_post_meta( $post_id,'product_desc_en', wp_kses_post( $_POST['product_desc_en'] ) );
}
add_action( 'save_post', 'enginfo_save_meta_box' );

However, the custom created field that supposed to be going only into the newly created metabox, will always show up in the default "custom field". And this happens to all post type. As shown below, What could possibly be the issue here?

To hide and not show your custom fields there in default box, please prefix your custom fields with underscore _ , so product_desc_en will become _product_des_en

I mean the names of your custom fields should be prefixed with underscore and WordPress default custom metabox will ignore them and not show in WordPress default GUI, but you can use and display them in your own custom metaboxes by calling with there new Underscore prefixed names.