Magento 2: Add custom value to $parent in checkout summary

I want to add a custom value to the items in the order summary during checkout ($parent).
I followed all the other howtos on how to do it:

Create plugin for MagentoCheckoutModelDefaultConfigProvider:

etc/di.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="MagentoCheckoutModelDefaultConfigProvider">
        <plugin name="my_config_provider" type="MyModulePluginConfigProviderPlugin" />
    </type>
</config>

This is my plugin:

namespace MyModulePlugin;

class ConfigProviderPlugin extends MagentoFrameworkModelAbstractModel
{
    /**
     * @param MagentoCheckoutModelDefaultConfigProvider $subject
     * @param array $result
     *
     * @return array
     */
    public function afterGetConfig(MagentoCheckoutModelDefaultConfigProvider $subject, array $result): array
    {
        foreach ($result['quoteItemData'] as $key => $item) {
            $result['quoteItemData'][$key]['display_price'] = $item['row_total'];
            $result['quoteItemData'][$key]['display_price'] += $item['tax_amount'];
        }

        return $result;
    }
}

This is a simplyfied version of my plugin. I made sure this is properly implemented. The plugin is being called.

But it seems, that none of the modifications I make to $result['quoteItemData'] make it to the frontend. The new display_price value is supposed to show up in vendormagentomodule-checkoutviewfrontendwebtemplatesummaryitemdetails.html (but it does not).

<span class="qty-price" data-bind="text: getFormattedPrice($parent.display_price)"></span>

Also when I add a debug to the template it shows that $parent is unchanged:

<pre data-bind="text: JSON.stringify(ko.toJS($parent), null, 2)"></pre>

What am I doing wrong here?
Thank you

EDIT:

The weird part is that NOTHING I do to quoteItemData is being reflected in the frontend.

Just for exploring purposes I modified the core file vendormagentomodule-checkoutModelDefaultConfigProvider.php. I added an extra entry to that array in getQuoteItemData. It does not get to the frontend whatsoever.