This issue is still not fixed but there is a workaround from @dcabrejasazagra here

Workaround:

Override vendor/magento/module-checkout/view/frontend/web/js/model/shipping-save-processor/default.js into your theme

magento/app/design/frontend/Vendor/Theme/Magento_Checkout/web/js/model/shipping-save-processor/default.js

Inside this file you will find fullScreenLoader.stopLoader(); called on done and fail methods

So you need to replace them with fullScreenLoader.stopLoader(true); which will force stop the loader and resolve the issue.

stopLoader() method is defined in vendor/magento/module-checkout/view/frontend/web/js/model/full-screen-loader.js

Edit: I assume that some component fails on the checkout and trigger .fail() method but the error processor is not handling errors correctly and display errors and hides the loader.

You can set your custom attribute inside checkout page summary block using below way,

app/code/{Package}/{Module}/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">    
    <!-- Set Custom attribute in chekcout page summary block -->
    <type name="Magento\Checkout\Model\DefaultConfigProvider">
        <plugin name="default_config_provider" type="Package\Module\Model\DefaultConfigProvider"/>
    </type>
</config>

app/code/{Package}/{Module}/Model/DefaultConfigProvider.php file

<?php
namespace Package\Module\Model;

use Magento\Checkout\Model\Session as CheckoutSession;
use Magento\Quote\Api\CartItemRepositoryInterface as QuoteItemRepository;

class DefaultConfigProvider
{
    private $checkoutSession;
    private $quoteItemRepository;
    protected $scopeConfig;

    public function __construct(
        CheckoutSession $checkoutSession,
        QuoteItemRepository $quoteItemRepository,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
    ) {
        $this->_scopeConfig = $scopeConfig;
        $this->checkoutSession = $checkoutSession;
        $this->quoteItemRepository = $quoteItemRepository;
    }

    public function afterGetConfig(\Magento\Checkout\Model\DefaultConfigProvider $subject, $result)
    {        
        $quoteId = $this->checkoutSession->getQuote()->getId();
        if ($quoteId) {            
            $itemOptionCount = count($result['totalsData']['items']);
            $quoteItems = $this->quoteItemRepository->getList($quoteId);

            $isbnOptions = array();
            foreach ($quoteItems as $index => $quoteItem) {
                $quoteItemId = $quoteItem['item_id'];
                $isbnOptions[$quoteItemId] = $quoteItem['isbn'];               
            }

            for($i=0;$i<$itemOptionCount;$i++){
                $quoteParentId = $result['totalsData']['items'][$i]['item_id'];                    
                $currentOption = array();
                $currentOption = json_decode($result['totalsData']['items'][$i]['options'],true);

                $newOption = $isbnOptions[$quoteParentId];

                //for set new option inside current option tab
                if(count($newOption) > 0 && is_array($newOption)){                        
                    foreach($newOption as $key=>$value){
                        if( !is_array($value) ){
                            $currentOption[] = array('value'=>$value,'label'=>$key);
                        }
                    }
                    $result['totalsData']['items'][$i]['options'] = json_encode($currentOption);
                }                    
            }
        }
        return $result;
    }
}