# reduce stock filter
function stock_change_email_log( $order ) {
	$email = 'youemail@domain';
	$subject = 'Stock is changed - Log';
	$product_details = array('<h4>Product changed details:</h4>');
	$order_items = $order->get_items();
	foreach( $order_items as $product ) {
		$product_details[] = 'UN: ' . $product['qty'] . '  |  (' . $product['product_id'] . ') ' . $product['name'];
	}
	$product_list = implode( '<br>', $product_details );
	wp_mail($email, $subject, $product_list);
}

add_action( 'woocommerce_reduce_order_stock', 'stock_change_email_log' );

# enable html content
function email_log_content_type(){
    return "text/html";
}
add_filter( 'wp_mail_content_type','email_log_content_type' );