/**
 * Modify members directory title for the current member type
 *
 * @param array $title title parts
 *
 * @return array
 */
function buddydev_modify_member_type_directory_title( $title ) {
 
    if ( ! bp_is_members_directory() ) {
        return $title;
    }
 
    $member_type = bp_get_current_member_type();
 
    if ( ! $member_type ) {
        return $title;
    }
 
    $type_object = bp_get_member_type_object( $member_type );
 
    $sep = apply_filters( 'document_title_separator', '-' );
    // we are reusing the buddypress()->pages->members->id page's title,
    // you can call get_post() over it and do any extra manipulation
    $title['title'] = $title['title'] . ' ' . $sep . ' ' . $type_object->labels['name'];
 
    return $title;
}
add_filter( 'document_title_parts', 'buddydev_modify_member_type_directory_title', 1000 );