Setting $order_item->setUnitPrice($price, TRUE);) stops cart items being merged, how to prevent this?

Drupal Commerce 2, with Drupal 9.

In the CART_ENTITY_ADD event, if i set $order_item->setUnitPrice($price, TRUE); identical items added to the cart get added as new order items, but I want them to be merged.

If I comment out this line in my code the items get merged as expected and their quantity is summed.

How do i set the unit price without the order items being treated as distinct order items?

Relevant code of the CART_ENTITY_ADD event:

public function addToCart(CartEntityAddEvent $event) {
    $order_item = $event->getOrderItem();
    // update order item
    $order_item->setUnitPrice($price, TRUE);
    $order_item->save();
}

Hmmmmmmm rather than order_item->save();, try using the cart manager

// get the cart
$cart_provider = \Drupal::service('commerce_cart.cart_provider');
$cart = $cart_provider->getCart('default');
// update the cart
$cart_manager = \Drupal::service('commerce_cart.cart_manager');
$order_item->setUnitPrice($price, TRUE);
$cart_manager->updateOrderItem($cart, $order_item);

_{Bonus: Some methods/functions you might find useful.}