magento-test/app/code/Magento/Downloadable/etc/webapi_soap/di.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Catalog\Helper\Product\ConfigurationPool">
        <arguments>
            <argument name="instancesByType" xsi:type="array">
                <item name="downloadable" xsi:type="object">Magento\Downloadable\Helper\Catalog\Product\Configuration</item>
            </argument>
        </arguments>
    </type>
</config>