diff --git a/vendor/magento/module-configurable-product/Model/Plugin/Frontend/UsedProductsCache.php b/vendor/magento/module-configurable-product/Model/Plugin/Frontend/UsedProductsCache.php
--- a/vendor/magento/module-configurable-product/Model/Plugin/Frontend/UsedProductsCache.php
+++ b/vendor/magento/module-configurable-product/Model/Plugin/Frontend/UsedProductsCache.php
@@ -125,6 +125,12 @@
             $product->getStoreId(),
             $this->customerSession->getCustomerGroupId(),
         ];
+        // DO
+        $keyParts[] = $product->getSku();
+        if (isset($_SERVER['REQUEST_URI']) && str_contains($_SERVER['REQUEST_URI'], 'fastorder')) {
+            $keyParts[] = 'fastorder';
+        }
+        // DO
         if ($requiredAttributeIds !== null) {
             sort($requiredAttributeIds);
             $keyParts[] = implode('', $requiredAttributeIds);