Relationship field ignores "max_clone" setting

Hi,

Why the relationship field ignores the "max_clone" setting?
"+ Add" button and drag/sort handle still there.
Maybe some other settings needs to be specified?

add_action( 'mb_relationships_init', 'tickets_to_event_relationship' );

function tickets_to_event_relationship() {
    MB_Relationships_API::register( [
        'id'   => 'tickets-to-event',
        'from' => [
            'object_type'   => 'post',
            'post_type'     => 'ticket',
            'meta_box' => [
                'context'   => 'normal',
                'priority'  => 'high',
            ],
        ],
        'to'   => [
            'object_type'   => 'post',
            'post_type'     => 'poster',
            'meta_box' => [
                'context'   => 'normal',
                'priority'  => 'high',
                'closed'    => true,
            ],
            'field' => [
                <strong>'max_clone' => '1',</strong>
                'required'  => true,
            ],
        ],
    ] );
}