Magento 2 - Error with custom plugin when adding bundle product

Question

My requirement is that certain kind of bundle products are not allowed to be added more than once in an order. I am trying to do so by intercepting the Add To Cart API method using a Plugin.

Here's my code:

di.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <!-- Interceptor (plugin) for Add To Cart action -->
    <!-- Prevent adding multiple products of the same kind to cart -->
    <type name="Magento\Quote\Api\CartItemRepositoryInterface">
        <plugin name="interceptAddToCart"
                type="MyCompany\MyModule\Plugin\CartItemRepositoryPlugin"
                sortOrder="1"
                disabled="false"/>
    </type>
</config>

Plugin file:

<?php

namespace MyCompany\MyModule\Plugin;

use Magento\Framework\Exception\CouldNotSaveException;
use Magento\Catalog\Api\ProductRepositoryInterface;
use Magento\Quote\Api\CartItemRepositoryInterface;

class CartItemRepositoryPlugin
{
    protected $_productRepositoryInterface;
    protected $_cartItemRepositoryInterface;

    /**
     * Plugin constructor.
     *
     * @param \Magento\Catalog\Api\ProductRepositoryInterface $productRepositoryInterface
     * @param \Magento\Quote\Api\CartItemRepositoryInterface $cartItemRepositoryInterface
     */
    public function __construct(
        ProductRepositoryInterface $productRepositoryInterface,
        CartItemRepositoryInterface $cartItemRepositoryInterface
    ) {
        $this->_productRepositoryInterface = $productRepositoryInterface;
        $this->_cartItemRepositoryInterface = $cartItemRepositoryInterface;
    }

    /**
     * beforeSave
     *
     * @param $subject
     * @param \Magento\Quote\Api\Data\CartItemInterface $cartItem The item.
     * @return array
     * @throws \Magento\Framework\Exception\NoSuchEntityException The specified cart does not exist.
     * @throws \Magento\Framework\Exception\CouldNotSaveException The specified item could not be saved to the cart.
     * @throws \Magento\Framework\Exception\InputException The specified item or cart is not valid.
     */
    public function beforeSave($subject, $cartItem)
    {
        // For debugging purposes I removed the logic and simply returned the cart item.
        return [$cartItem];
    }
}

For adding products to cart, I am using Magento's default REST API.

[POST] {{base_url}}rest/V1/carts/mine/items

Parameters:

{
    "cart_item": {
        "quote_id": "73",
        "sku": "product_sku",
        "qty": 1,
        "product_option": {
            "extension_attributes": {
                "bundle_options": [
                    {
                        "option_id": 30,
                        "option_qty": 1,
                        "option_selections": [41]
                    },
                    {
                        "option_id": 31,
                        "option_qty": 1,
                        "option_selections": [42]
                    },
                    {
                        "option_id": 32,
                        "option_qty": 1,
                        "option_selections": [43]
                    },
                    {
                        "option_id": 33,
                        "option_qty": 1,
                        "option_selections": [44]
                    }
                ]
            }
        }
    }
}

Now when I hit the Add To Cart API, I see this message:

{
    "message": "Please specify product option(s).",
    "trace": "#0 {path}/vendor/magento/module-quote/Model/Quote/Item/CartItemPersister.php(84): Magento\\Quote\\Model\\Quote->addProduct(Object(Magento\\Catalog\\Model\\Product\\Interceptor), Object(Magento\\Framework\\DataObject))\n#1 {path}/vendor/magento/module-quote/Model/QuoteRepository/SaveHandler.php(69): Magento\\Quote\\Model\\Quote\\Item\\CartItemPersister->save(Object(Magento\\Quote\\Model\\Quote), Object(Magento\\Quote\\Model\\Quote\\Item))\n#2 {path}/vendor/magento/module-quote/Model/QuoteRepository.php(161): Magento\\Quote\\Model\\QuoteRepository\\SaveHandler->save(Object(Magento\\Quote\\Model\\Quote))\n#3 {path}/vendor/magento/module-quote/Model/Quote/Item/Repository.php(96): Magento\\Quote\\Model\\QuoteRepository->save(Object(Magento\\Quote\\Model\\Quote))\n#4 {path}/vendor/magento/framework/Interception/Interceptor.php(146): Magento\\Quote\\Model\\Quote\\Item\\Repository->save(Object(Magento\\Quote\\Model\\Quote\\Item))\n#5 {path}/var/generation/Magento/Quote/Model/Quote/Item/Repository/Interceptor.php(26): Magento\\Quote\\Model\\Quote\\Item\\Repository\\Interceptor->___callPlugins('save', Array, Array)\n#6 [internal function]: Magento\\Quote\\Model\\Quote\\Item\\Repository\\Interceptor->save(Object(Magento\\Quote\\Model\\Quote\\Item))\n#7 {path}/vendor/magento/module-webapi/Controller/Rest.php(307): call_user_func_array(Array, Array)\n#8 {path}/vendor/magento/module-webapi/Controller/Rest.php(216): Magento\\Webapi\\Controller\\Rest->processApiRequest()\n#9 {path}/var/generation/Magento/Webapi/Controller/Rest/Interceptor.php(24): Magento\\Webapi\\Controller\\Rest->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#10 {path}/vendor/magento/framework/App/Http.php(135): Magento\\Webapi\\Controller\\Rest\\Interceptor->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#11 {path}/vendor/magento/framework/App/Bootstrap.php(258): Magento\\Framework\\App\\Http->launch()\n#12 {path}/pub/index.php(37): Magento\\Framework\\App\\Bootstrap->run(Object(Magento\\Framework\\App\\Http))\n#13 {main}"
}

If I create a fresh new quote and repeat this process, the outcome is the same.

If I disable my Plugin, either by entirely removing it, or by setting disabled="true" in di.xml, everything works fine.

What could be going wrong?