I am facing this issue in the backend admin, in the customer Information, & Shopping Cart tab. It looks like the getQuote() function returns all cart items if no active quote is found for customer id.

Screenshot :

Please follow the below steps to fix this issue :=>

Please create the following extension with a plugin

1) Vendor/Module/etc/adminhtml/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">
    <preference for="MagentoCustomerBlockAdminhtmlEditTabCart" type="VendorModuleRewriteCustomerBlockAdminhtmlEditTabCart"/>
    <type name="MagentoSalesBlockAdminhtmlShoppingCartsTab">
        <plugin sortOrder="1" name="FixShoppingCartsTab"
                type="VendorModulePluginBlockSalesShoppingCartsTab"/>
    </type>
</config>

2)Vendor/Module/Rewrite/Customer/Block/Adminhtml/Edit/Tab/Cart.php

<?php
namespace VendorModuleRewriteCustomerBlockAdminhtmlEditTab;

use MagentoBackendBlockWidgetGridExtended;
use MagentoFrameworkExceptionNoSuchEntityException;

class Cart extends MagentoCustomerBlockAdminhtmlEditTabCart
{
    /**
     * Added fix when customer do not have any active quote
     * Magento 2 issue: https://github.com/magento/magento2/issues/26437
     * Pull request: https://github.com/magento/magento2/pull/26489
     */
    protected function _prepareCollection()
    {
        $quote = $this->getQuote();

        if ($quote && $quote->getId()) {
            $collection = $quote->getItemsCollection(false);
            $collection->addFieldToFilter('parent_item_id', ['null' => true]);
        } else {
            $collection = $this->_dataCollectionFactory->create();
        }

        $this->setCollection($collection);

        return Extended::_prepareCollection();
    }

    protected function getQuote()
    {
        if (null === $this->quote) {
            $customerId = $this->getCustomerId();
            $storeIds = $this->_storeManager->getWebsite($this->getWebsiteId())->getStoreIds();

            try {
                $this->quote = $this->quoteFactory->create()->setSharedStoreIds($storeIds)->loadByCustomer($customerId);
            } catch (NoSuchEntityException $e) {
                $this->quote = $this->quoteFactory->create()->setSharedStoreIds($storeIds);
            }
        }
        return $this->quote;
    }
}

3)  Vendor/Module/Plugin/Block/SalesShoppingCartsTab.php

<?php
namespace VendorModulePluginBlock;

use MagentoSalesBlockAdminhtmlShoppingCartsTab;

class SalesShoppingCartsTab
{
    public function afterGetTabUrl(ShoppingCartsTab $subject, $result)
    {
        return $subject->getUrl('customer/*/carts', ['_current' => true]);
    }
}

That’s it! You are done.

Note:  This will only work for Magento 2.3.3 and higher.

Please share your thoughts if this article helpful to you.

Thanks.