This shortcode creates a custom woocommerce order type by an acf field called custom_product_popularity.

functions.php

<?php
// creating custom woocommerce order by an acf field called custom_product_popularity
function leoshop_add_new_postmeta_orderby( $sortby ) {
  	$sortby['custom_popularity'] = __( 'Sortieren nach benutzerdefinierten', 'hello-theme-child');
  	return $sortby;
}
add_filter( 'woocommerce_default_catalog_orderby_options', 'leoshop_add_new_postmeta_orderby' );
add_filter( 'woocommerce_catalog_orderby', 'leoshop_add_new_postmeta_orderby' );

function leoshop_add_postmeta_ordering_args( $sort_args ) {
 
	$cw_orderby_value = isset( $_GET['orderby'] ) ? wc_clean( $_GET['orderby'] ) :
	apply_filters( 'woocommerce_default_catalog_orderby', get_option('woocommerce_default_catalog_orderby'));
	
	switch($cw_orderby_value) {
		case 'custom_popularity':
			$sort_args['orderby'] = 'meta_value_num';
			$sort_args['order'] = 'ASC';
			$sort_args['meta_key'] = 'custom_product_popularity';
			break;
	}

	return $sort_args;
}
add_filter('woocommerce_get_catalog_ordering_args', 'leoshop_add_postmeta_ordering_args');