coz hidden outofstock products is disabled in woocommerce
1: you can enable this option
OR
2: you can replace
$hide_oos = ‘yes’ == get_option( ‘woocommerce_hide_out_of_stock_items’ );
TO:
$hide_oos = true;
in /plugins/force-default-variant-for-woocommerce/includes/variations.php LINE 88
