Memory leak when using OrderItemCollection

Question

I'm trying to execute some backend logic via ajax that requires using the Magento\Sales\Model\ResourceModel\Order\Item\CollectionFactory collection factory.

When I use ->getColumnValues('<some column>'), I run out of memory if I don't set the php memory limit at 2GB. Most importantly, the memory doesn't clean itself out after code execution.

Before execution:

During execution:

After execution:

The controller in question:

<?php


namespace ...

use Magento\Sales\Model\ResourceModel\Order\Item\CollectionFactory as OrderItemCollectionFactory;

class Statistics extends Action
{
    
    public function __construct(
        ...
        OrderItemCollectionFactory $orderItemCollectionFactory,
        ...
    )
    {
        parent::__construct($context);
        ...
        $this->_orderItemCollectionFactory = $orderItemCollectionFactory;
        ...
    }

    public function execute()
    {
        $product_ids = $this->getBrandProductIds();
        $json = $this->_jsonFactory->create();
        return $json->setData([
            'test' => $this->_orderItemCollectionFactory->create()
                ->addFieldToFilter('product_id', ['in' => $product_ids])
                ->getColumnValues('order_id')
        ]);
    }
}