wp_user-query: relation OR and sort

I want a list of all users who meet either condition1 or condition2. The list is sorted by lastname. For the query I use the following code:

$args = array(
    'meta_query'    => array(
        'relation' => 'OR',
        0 => array(
            'key'       => 'user_thema1',
            'value'     => $kate_name,
            'compare'   => '='
        ),
        1 => array(
            'key'     => 'user_thema2',
            'value'   => $kate_name,
            'compare' => '='
        )
    ),
    'meta_key'      => 'last_name',
    'order'         => 'ASC',
    'orderby'       => 'meta_value'
);
$user_query = new WP_User_Query( $args );

All users are displayed. If a condition is fulfilled, the list is unsorted. If no condition is satisfied, the list is sorted.

If I take only one condition or two conditions with AND connect, the result is OK.

What’s wrong?