woocommerce: fix "out of stock" message, if more than 30 variants are existing.

functions.php

<?php 

function custom_wc_ajax_variation_threshold( $qty, $product ) {
    return sizeof( $product->get_children() );
}

add_filter( 'woocommerce_ajax_variation_threshold', 'custom_wc_ajax_variation_threshold', 10, 2 );