With WooCommerce 3+ introducing new API to fetch the order and it's details, a lot of things have changed and many things break as well.
Consider the following code in my plugin:
$order = wc_get_order($order_id);
$id= 27;
var_dump($order->get_item($id));
which gives me bool(false). I have checked the database and the order and the item does exist.
Also
var_dump($order)
does return the entire order object with all the items.
So basically, only the function get_item
does not seem to work.