Woocommerce CSV导出未显示值

I've noticed a strange behaviour of one of my functions. Just take a look. Using first function I'm displaying some custom fields data in the backend:

add_filter( 'manage_edit-shop_order_columns', 'MY_COLUMNS_FUNCTION', 10 );
function MY_COLUMNS_FUNCTION( $columns ) {
    $new_columns = ( is_array( $columns ) ) ? $columns : array();
    unset( $new_columns['order_actions'] );
    //edit this for you column(s)
    //all of your columns will be added before the actions column
    $new_columns['product_name'] = 'Product';
    $new_columns['authors_income'] = 'Author';

    $new_columns['order_actions'] = $columns['order_actions'];

    return $new_columns;
}

add_action( 'manage_shop_order_posts_custom_column', 'MY_COLUMNS_VALUES_FUNCTION', 2 );
function MY_COLUMNS_VALUES_FUNCTION( $column ) {
    global $post;
    $order = new WC_Order( $post->ID );
    $items = $order->get_items();

    //start editing, I was saving my fields for the orders as custom post meta
    //if you did the same, follow this code
    if ( $column == 'authors_income' ) {
        foreach ( $items as $item ) {

            echo $item['PLN-dla-autora'];
            echo ' zł';

        }
    }

    if ( $column == 'product_name' ) {
        foreach ( $items as $item ) {

            echo $item['name'];

        }
    }

}

add_filter( "manage_edit-shop_order_sortable_columns", 'MY_COLUMNS_SORT_FUNCTION' );
function MY_COLUMNS_SORT_FUNCTION( $columns ) {
    $custom = array(
        //start editing

        'authors_income'    => 'PLN-dla-autora',
        'product_name'    => 'name'

        //stop editing
    );
    return wp_parse_args( $custom, $columns );

Everything works there like a charm. Then I have second function, which somehow does the same, yet it will print the data into the CSV file. Unfortunatelly, this code doesnt work (it display only column header, no data included). I dont see any major differences between those functions so why is that?

function wc_csv_export_modify_column_headers( $column_headers ) { 

    $new_headers = array(
        'author_income' => 'PLN dla autora',

        // add other column headers here in the format column_key => Column Name
    );

    return array_merge( $column_headers, $new_headers );
}
add_filter( 'wc_customer_order_csv_export_order_headers', 'wc_csv_export_modify_column_headers' );
// set the data for each for custom columns
function wc_csv_export_modify_row_data( $order_data, $order ) {

    $custom_data = array(
        'author_income' => get_post_meta( $order->id, 'PLN dla autora', true ),

        // add other row data here in the format column_key => data
    );

    return array_merge( $order_data, $custom_data );

}
add_filter( 'wc_customer_order_csv_export_order_row', 'wc_csv_export_modify_row_data', 10, 2 );