WP_User_Query search seems bugged

Hi,
I’m trying to create a user search query, which would search by all user colums (login, nicename, email, url) and by metadata.
This is my code:

$args = array(
			'fields'				=> 'all_with_meta',
			'search'				=> '*'.$search_string.'*',
			'search_columns'	=> array(
				'user_login',
				'user_nicename',
				'user_email',
				'user_url',
			),
			'meta_query' => array(
				'relation'		=> 'OR',
				array(
					'key'			=> 'first_name',
					'value'		=> $search_string,
					'compare'	=> 'LIKE'
				),
				array(
					'key'			=> 'last_name',
					'value'		=> $search_string,
					'compare'	=> 'LIKE'
				),
			),
		);
		$user_query = new WP_User_Query( $args );

And it seems like search_columns are ignored, and this is the resulting query:

[query_fields] => DISTINCT SQL_CALC_FOUND_ROWS wp_users.*
    [query_from] => FROM wp_users INNER JOIN wp_usermeta ON ( wp_users.ID = wp_usermeta.user_id )
    [query_where] => WHERE 1=1 AND (
  ( wp_usermeta.meta_key = 'first_name' AND CAST(wp_usermeta.meta_value AS CHAR) LIKE '%parent%' )
  OR
  ( wp_usermeta.meta_key = 'last_name' AND CAST(wp_usermeta.meta_value AS CHAR) LIKE '%parent%' )
) AND (display_name LIKE 'parent')
    [query_orderby] => ORDER BY user_login ASC
    [query_limit] =>

So basically search is summed up to this, every time:
AND (display_name LIKE ‘parent’)

Whereas I would expect it to be:
OR (user_login LIKE 'parent') OR (user_nicename LIKE 'parent') OR (user_email LIKE 'parent') OR (user_url LIKE 'parent')

Any ideas?

Thanks!