diff -Nuar a/vendor/magento/module-catalog/Observer/CategoryProductIndexer.php b/vendor/magento/module-catalog/Observer/CategoryProductIndexer.php
index ca87efaa874..b9b0a205ac4 100644
--- a/vendor/magento/module-catalog/Observer/CategoryProductIndexer.php
+++ b/vendor/magento/module-catalog/Observer/CategoryProductIndexer.php
@@ -8,6 +8,7 @@ declare(strict_types=1);
 namespace Magento\Catalog\Observer;

 use Magento\Catalog\Model\Indexer\Category\Product\Processor;
+use Magento\Catalog\Model\Indexer\Category\Flat\State as FlatState;
 use Magento\Framework\Event\Observer;
 use Magento\Framework\Event\ObserverInterface;

@@ -22,11 +23,21 @@ class CategoryProductIndexer implements ObserverInterface
     private $processor;

     /**
+     * @var FlatState
+     */
+    private $flatState;
+
+    /**
      * @param Processor $processor
+     * @param FlatState $flatState
      */
-    public function __construct(Processor $processor)
+    public function __construct(
+        Processor $processor,
+        FlatState $flatState
+    )
     {
         $this->processor = $processor;
+        $this->flatState = $flatState;
     }

     /**
@@ -35,7 +46,7 @@ class CategoryProductIndexer implements ObserverInterface
     public function execute(Observer $observer): void
     {
         $productIds = $observer->getEvent()->getProductIds();
-        if (!empty($productIds) && $this->processor->isIndexerScheduled()) {
+        if (!empty($productIds) && $this->processor->isIndexerScheduled() && $this->flatState->isFlatEnabled()) {
             $this->processor->markIndexerAsInvalid();
         }
     }