diff --git a/vendor/magento/module-inventory-import-export/Plugin/Import/SourceItemImporter.php b/vendor/magento/module-inventory-import-export/Plugin/Import/SourceItemImporter.php
index d1c9a3e02c83..02bda87de4d1 100644
--- a/vendor/magento/module-inventory-import-export/Plugin/Import/SourceItemImporter.php
+++ b/vendor/magento/module-inventory-import-export/Plugin/Import/SourceItemImporter.php
@@ -7,12 +7,24 @@
 
 namespace Magento\InventoryImportExport\Plugin\Import;
 
-use Magento\CatalogImportExport\Model\StockItemImporterInterface;
-use Magento\InventoryApi\Api\Data\SourceItemInterfaceFactory;
+use Magento\CatalogImportExport\Model\Import\Product\SkuProcessor;
+use Magento\CatalogImportExport\Model\StockItemProcessorInterface;
+use Magento\Framework\App\ResourceConnection;
+use Magento\Framework\Exception\CouldNotSaveException;
+use Magento\Framework\Exception\InputException;
+use Magento\Framework\Validation\ValidationException;
+use Magento\Inventory\Model\ResourceModel\SourceItem;
 use Magento\InventoryApi\Api\Data\SourceItemInterface;
+use Magento\InventoryApi\Api\Data\SourceItemInterfaceFactory;
 use Magento\InventoryApi\Api\SourceItemsSaveInterface;
 use Magento\InventoryCatalogApi\Api\DefaultSourceProviderInterface;
+use Magento\InventoryCatalogApi\Model\IsSingleSourceModeInterface;
 
+/**
+ * Assigning products to default source
+ *
+ * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
+ */
 class SourceItemImporter
 {
     /**
@@ -36,57 +48,116 @@ class SourceItemImporter
      */
     private $defaultSource;
 
+    /**
+     * @var IsSingleSourceModeInterface
+     */
+    private $isSingleSourceMode;
+
+    /**
+     * @var ResourceConnection
+     */
+    private $resourceConnection;
+
+    /**
+     * @var SkuProcessor
+     */
+    private $skuProcessor;
+
     /**
      * StockItemImporter constructor
      *
      * @param SourceItemsSaveInterface $sourceItemsSave
      * @param SourceItemInterfaceFactory $sourceItemFactory
      * @param DefaultSourceProviderInterface $defaultSourceProvider
+     * @param IsSingleSourceModeInterface $isSingleSourceMode
+     * @param ResourceConnection $resourceConnection
+     * @param SkuProcessor $skuProcessor
      */
     public function __construct(
         SourceItemsSaveInterface $sourceItemsSave,
         SourceItemInterfaceFactory $sourceItemFactory,
-        DefaultSourceProviderInterface $defaultSourceProvider
+        DefaultSourceProviderInterface $defaultSourceProvider,
+        IsSingleSourceModeInterface $isSingleSourceMode,
+        ResourceConnection $resourceConnection,
+        SkuProcessor $skuProcessor
     ) {
         $this->sourceItemsSave = $sourceItemsSave;
         $this->sourceItemFactory = $sourceItemFactory;
         $this->defaultSource = $defaultSourceProvider;
+        $this->isSingleSourceMode = $isSingleSourceMode;
+        $this->resourceConnection = $resourceConnection;
+        $this->skuProcessor = $skuProcessor;
     }
 
     /**
      * After plugin Import to import Stock Data to Source Items
      *
-     * @param StockItemImporterInterface $subject
-     * @param null $result
+     * @param StockItemProcessorInterface $subject
+     * @param mixed $result
      * @param array $stockData
-     * @throws \Magento\Framework\Exception\CouldNotSaveException
-     * @throws \Magento\Framework\Exception\InputException
-     * @throws \Magento\Framework\Validation\ValidationException
+     * @param array $importedData
      * @return void
-     * @see StockItemImporterInterface::import()
+     * @throws CouldNotSaveException
+     * @throws InputException
+     * @throws ValidationException
      *
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
-    public function afterImport(
-        StockItemImporterInterface $subject,
+    public function afterProcess(
+        StockItemProcessorInterface $subject,
         $result,
-        array $stockData
-    ) {
+        array $stockData,
+        array $importedData
+    ): void {
         $sourceItems = [];
+        $skusHavingDefaultSource = $this->getSkusHavingDefaultSource(array_keys($stockData));
+
         foreach ($stockData as $sku => $stockDatum) {
-            $inStock = (isset($stockDatum['is_in_stock'])) ? intval($stockDatum['is_in_stock']) : 0;
-            $qty = (isset($stockDatum['qty'])) ? $stockDatum['qty'] : 0;
-            /** @var SourceItemInterface $sourceItem */
+            $isNewSku = !array_key_exists(strtolower((string)$sku), $this->skuProcessor->getOldSkus());
+            $isQtyExplicitlySet = $importedData[$sku]['qty'] ?? false;
+
+            $inStock = $stockDatum['is_in_stock'] ?? 0;
+            $qty = $stockDatum['qty'] ?? 0;
             $sourceItem = $this->sourceItemFactory->create();
             $sourceItem->setSku((string)$sku);
             $sourceItem->setSourceCode($this->defaultSource->getCode());
             $sourceItem->setQuantity((float)$qty);
-            $sourceItem->setStatus($inStock);
-            $sourceItems[] = $sourceItem;
+            $sourceItem->setStatus((int)$inStock);
+
+            //Prevent existing products to be assigned to `default` source, when `qty` is not explicitly set.
+            if ($isNewSku
+                || $isQtyExplicitlySet
+                || $this->isSingleSourceMode->execute()
+                || in_array($sourceItem->getSku(), $skusHavingDefaultSource, true)) {
+                $sourceItems[] = $sourceItem;
+            }
         }
         if (count($sourceItems) > 0) {
             /** SourceItemInterface[] $sourceItems */
             $this->sourceItemsSave->execute($sourceItems);
         }
     }
+
+    /**
+     * Fetch product's skus having assigned to `default` source.
+     *
+     * @param array $listSku
+     * @return array
+     */
+    private function getSkusHavingDefaultSource(array $listSku): array
+    {
+        $connection = $this->resourceConnection->getConnection();
+        $select = $connection->select()->from(
+            $this->resourceConnection->getTableName(SourceItem::TABLE_NAME_SOURCE_ITEM),
+            [SourceItemInterface::SKU]
+        )->where(
+            SourceItemInterface::SKU . ' IN (?)',
+            $listSku
+        )->where(
+            SourceItemInterface::SOURCE_CODE . ' = ?',
+            $this->defaultSource->getCode()
+        );
+
+        return $connection->fetchCol($select);
+    }
 }
diff --git a/vendor/magento/module-inventory-import-export/etc/di.xml b/vendor/magento/module-inventory-import-export/etc/di.xml
index 6f0657decfd3..7d031a68149f 100644
--- a/vendor/magento/module-inventory-import-export/etc/di.xml
+++ b/vendor/magento/module-inventory-import-export/etc/di.xml
@@ -27,9 +27,8 @@
         </arguments>
     </type>
 
-    <!-- Source Item Import Plugin -->
-    <type name="Magento\CatalogImportExport\Model\StockItemImporterInterface">
-        <plugin name="importStockItemDataForSourceItem" type="Magento\InventoryImportExport\Plugin\Import\SourceItemImporter" />
+    <type name="Magento\CatalogImportExport\Model\StockItemProcessorInterface">
+        <plugin name="importStockItemDataForSourceItem" type="Magento\InventoryImportExport\Plugin\Import\SourceItemImporter"/>
     </type>
     <!-- Prevent stock indexer to execute twice as it is run by default -->
     <type name="Magento\CatalogImportExport\Model\Import\Product\StockProcessor">