Code Snippet working on one of two sites

// unhook this function so it doesn't loop infinitely
remove_action('save_post', 'person_acf_custom_title');

// update the post, which calls save_post again
// Update post
$my_post = array();
$my_post['ID'] = get_the_ID();
$my_post['post_title'] = get_field( 'titel' )." ". get_field( 'vorname' )." ". get_field( 'nachname' );
$my_post['post_name'] = sanitize_title( $my_post['post_title'] );
// Update the post into the database
wp_update_post( $my_post );

// Register Custom Post Type
function person_post_type() {

$labels = array(
'name' => _x( 'Personen', 'Post Type General Name', 'text_domain' ),
'singular_name' => _x( 'Person', 'Post Type Singular Name', 'text_domain' ),
'menu_name' => __( 'Personen', 'text_domain' ),
'name_admin_bar' => __( '', 'text_domain' ),
'archives' => __( '', 'text_domain' ),
'attributes' => __( 'Item Attributes', 'text_domain' ),
'parent_item_colon' => __( 'Parent Item:', 'text_domain' ),
'all_items' => __( 'Alle Personen', 'text_domain' ),
'add_new_item' => __( 'Neue Person hinzufügen', 'text_domain' ),
'add_new' => __( 'Hinzufügen', 'text_domain' ),
'new_item' => __( 'Neue Person', 'text_domain' ),
'edit_item' => __( 'Person bearbeiten', 'text_domain' ),
'update_item' => __( 'Person aktualisieren', 'text_domain' ),
'view_item' => __( 'Person anschauen', 'text_domain' ),
'view_items' => __( 'Personen anschauen', 'text_domain' ),
'search_items' => __( 'Personen suchen', 'text_domain' ),
'not_found' => __( 'Not found', 'text_domain' ),
'not_found_in_trash' => __( 'Not found in Trash', 'text_domain' ),
'featured_image' => __( 'Profilbild', 'text_domain' ),
'set_featured_image' => __( 'Profilbild festlegen', 'text_domain' ),
'remove_featured_image' => __( 'Profilbild entfernen', 'text_domain' ),
'use_featured_image' => __( 'Als Profilbild nutzen', 'text_domain' ),
'insert_into_item' => __( 'Insert into item', 'text_domain' ),
'uploaded_to_this_item' => __( 'Uploaded to this item', 'text_domain' ),
'items_list' => __( 'Items list', 'text_domain' ),
'items_list_navigation' => __( 'Items list navigation', 'text_domain' ),
'filter_items_list' => __( 'Filter items list', 'text_domain' ),
);
$args = array(
'label' => __( 'Person', 'text_domain' ),
'description' => __( 'Personen Register', 'text_domain' ),
'labels' => $labels,
'supports' => array( 'thumbnail' , 'editor' ),
'taxonomies' => array( 'mandate_categories'),
'hierarchical' => true,
'public' => true,
'show_ui' => true,
'show_in_menu' => true,
'menu_position' => 21,
'menu_icon' => 'dashicons-id-alt',
'show_in_admin_bar' => false,
'show_in_nav_menus' => true,
'can_export' => true,
'has_archive' => true,
'exclude_from_search' => false,
'publicly_queryable' => true,
'capability_type' => 'post',
'rewrite' => array( 'slug' => 'personen' ),
);
register_post_type( 'person', $args );

}
add_action( 'init', 'person_post_type', 0 );
}

function person_taxonomy() {
register_taxonomy(
'mandate_categories', //The name of the taxonomy. Name should be in slug form (must not contain capital letters or spaces).
'person', //post type name
array(
'hierarchical' => true,
'label' => 'Mandate', //Display name
'query_var' => true,
'rewrite' => array(
'slug' => 'mandate', // This controls the base slug that will display before each term
'with_front' => false // Don't display the category base before
)
)
);
}
add_action( 'init', 'person_taxonomy');

function person_acf_custom_title( $post_id ){
if ( ! wp_is_post_revision( $post_id ) and get_post_type() == 'person' and get_the_title() != get_field( 'titel' )." ". get_field( 'vorname' )." ". get_field( 'nachname' ) ){

// unhook this function so it doesn't loop infinitely
remove_action('save_post', 'person_acf_custom_title');

// update the post, which calls save_post again
// Update post
$my_post = array();
$my_post['ID'] = get_the_ID();
$my_post['post_title'] = get_field( 'titel' )." ". get_field( 'vorname' )." ". get_field( 'nachname' );
$my_post['post_name'] = sanitize_title( $my_post['post_title'] );
// Update the post into the database
wp_update_post( $my_post );