Purchasing a downloadable product as guest then creating an account on the onepagesuccess step doesn't link product with account

app/code/Magento/Downloadable/Observer/UpdateLinkPurchasedObserver.php

@@ -0,0 +1,80 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\Downloadable\Observer;
+
+use Magento\Framework\Event\ObserverInterface;
+
+/**
+ * Assign Downloadable links to customer created after issuing guest order.
+ */
+class UpdateLinkPurchasedObserver implements ObserverInterface
+{
+    /**
+     * Core store config
+     * @var \Magento\Framework\App\Config\ScopeConfigInterface
+     */
+    private $scopeConfig;
+
+    /**
+     * @var \Magento\Downloadable\Model\ResourceModel\Link\Purchased\CollectionFactory
+     */
+    private $purchasedFactory;
+
+    /**
+     * @var \Magento\Framework\DataObject\Copy
+     */
+    private $objectCopyService;
+
+    /**
+     * @param \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
+     * @param \Magento\Downloadable\Model\ResourceModel\Link\Purchased\CollectionFactory $purchasedFactory
+     * @param \Magento\Framework\DataObject\Copy $objectCopyService
+     */
+    public function __construct(
+        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
+        \Magento\Downloadable\Model\ResourceModel\Link\Purchased\CollectionFactory $purchasedFactory,
+        \Magento\Framework\DataObject\Copy $objectCopyService
+    ) {
+        $this->scopeConfig = $scopeConfig;
+        $this->purchasedFactory = $purchasedFactory;
+        $this->objectCopyService = $objectCopyService;
+    }
+
+    /**
+     * Re-save order data after order update.
+     *
+     * @param \Magento\Framework\Event\Observer $observer
+     * @return $this
+     */
+    public function execute(\Magento\Framework\Event\Observer $observer)
+    {
+        $order = $observer->getEvent()->getOrder();
+
+        if (!$order->getId()) {
+            //order not saved in the database
+            return $this;
+        }
+
+        $purchasedLinks = $this->purchasedFactory->create()->addFieldToFilter(
+            'order_id',
+            ['eq' => $order->getId()]
+        );
+
+        foreach ($purchasedLinks as $linkPurchased) {
+            $this->objectCopyService->copyFieldsetToTarget(
+                \downloadable_sales_copy_order::class,
+                'to_downloadable',
+                $order,
+                $linkPurchased
+            );
+            $linkPurchased->save();
+        }
+
+        return $this;
+    }
+}

app/code/Magento/Downloadable/etc/events.xml

@@ -11,6 +11,7 @@
     </event>
     <event name="sales_order_save_after">
         <observer name="downloadable_observer" instance="Magento\Downloadable\Observer\SetLinkStatusObserver" />
+        <observer name="downloadable_observer_assign_customer" instance="Magento\Downloadable\Observer\UpdateLinkPurchasedObserver" />
     </event>
     <event name="sales_model_service_quote_submit_success">
         <observer name="checkout_type_onepage_save_order_after" instance="Magento\Downloadable\Observer\SetHasDownloadableProductsObserver" />