Fix bug 27490: Can't change customer group when placing an admin order

app/code/Magento/Quote/Observer/Backend/Quote/Address/CollectTotalsObserver.php

@@ -0,0 +1,81 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\Quote\Observer\Backend\Quote\Address;
+
+use Magento\Framework\App\Area;
+use Magento\Framework\App\State;
+use Magento\Framework\Event\ObserverInterface;
+use Magento\Quote\Observer\Frontend\Quote\Address\CollectTotalsObserver as FrontendCollectTotalsObserver;
+use Magento\Quote\Observer\Frontend\Quote\Address\VatValidator;
+
+/**
+ * Handle customer VAT number on collect_totals_before event of quote address.
+ *
+ * @SuppressWarnings(PHPMD.CookieAndSessionMisuse)
+ * @SuppressWarnings(PHPMD.TooManyFields)
+ */
+class CollectTotalsObserver extends FrontendCollectTotalsObserver implements ObserverInterface
+{
+    /**
+     * @var State
+     */
+    private $state;
+
+    /**
+     * Initialize dependencies.
+     *
+     * @param \Magento\Customer\Helper\Address $customerAddressHelper
+     * @param \Magento\Customer\Model\Vat $customerVat
+     * @param VatValidator $vatValidator
+     * @param \Magento\Customer\Api\Data\CustomerInterfaceFactory $customerDataFactory
+     * @param \Magento\Customer\Api\GroupManagementInterface $groupManagement
+     * @param \Magento\Customer\Api\AddressRepositoryInterface $addressRepository
+     * @param \Magento\Customer\Model\Session $customerSession
+     * @param State $state
+     */
+    public function __construct(
+        \Magento\Customer\Helper\Address $customerAddressHelper,
+        \Magento\Customer\Model\Vat $customerVat,
+        VatValidator $vatValidator,
+        \Magento\Customer\Api\Data\CustomerInterfaceFactory $customerDataFactory,
+        \Magento\Customer\Api\GroupManagementInterface $groupManagement,
+        \Magento\Customer\Api\AddressRepositoryInterface $addressRepository,
+        \Magento\Customer\Model\Session $customerSession,
+        State $state
+    ) {
+        parent::__construct(
+            $customerAddressHelper,
+            $customerVat,
+            $vatValidator,
+            $customerDataFactory,
+            $groupManagement,
+            $addressRepository,
+            $customerSession
+        );
+        $this->state = $state;
+    }
+
+    /**
+     * Conditions to change customer group
+     *
+     * @param int|null $groupId
+     * @return bool
+     */
+    protected function assignCustomerGroupConditions($groupId)
+    {
+        if ($groupId !== null
+            && !(
+                $this->state->getAreaCode() == Area::AREA_ADMINHTML
+                && $groupId == $this->groupManagement->getNotLoggedInGroup()->getId()
+            )) {
+            return true;
+        }
+
+        return false;
+    }
+}

app/code/Magento/Quote/Observer/Frontend/Quote/Address/CollectTotalsObserver.php

@@ -131,12 +131,23 @@ public function execute(\Magento\Framework\Event\Observer $observer)
             );
         }
 
-        if ($groupId !== null) {
+        if ($this->assignCustomerGroupConditions($groupId)) {
             $address->setPrevQuoteCustomerGroupId($quote->getCustomerGroupId());
             $quote->setCustomerGroupId($groupId);
             $this->customerSession->setCustomerGroupId($groupId);
             $customer->setGroupId($groupId);
             $quote->setCustomer($customer);
         }
     }
+
+    /**
+     * Conditions to change customer group
+     *
+     * @param int|null $groupId
+     * @return bool
+     */
+    protected function assignCustomerGroupConditions($groupId)
+    {
+        return $groupId !== null ? true : false;
+    }
 }