This is happening because in here - /include/func/func.ajax.php - at the end of each function the original template is called and it replaces whatever is there on the page. I still can't figure out why it is designed that way (4 versions already), it seems unnecessary thing to do. You can either comment out lines like this
Code:
return func_display('customer/main/product_details.tpl', $smarty, false);
Or replace with yoru own template