Sort WooCommerce Product Attribute Values on single product page. Details on: https://abdulawal.com/?p=1463

woocommerce-sort-attribute-values-in-dropdown.php

/**
 * Sort Attribute values in ascending order for any attribute
 * @param  array $args 
 * @return array return sorted $args array
 */
function abd_sort_wc_attribute_values( $args ){
	$get_options 		= $args['options'];
	asort( $get_options );
	$args['options'] 	= $get_options;
	return $args;
}

add_filter( 'woocommerce_dropdown_variation_attribute_options_args', 'abd_sort_wc_attribute_values', 10, 1 );