Filter the import to make sure we only update fields present in the import file.

 - magento/magento2#11968
 - Importing a import file to update customer data, results in entity fields being removed if the columns are not present on the imported file.

Author: dmanners

app/code/Magento/CustomerImportExport/Model/Import/Customer.php

@@ -277,6 +277,12 @@ protected function _saveCustomerEntities(array $entitiesToCreate, array $entitie
         return $this;
     }
 
+    /**
+     * Filter the entity that are being updated so we only change fields found in the importer file
+     *
+     * @param array $entitiesToUpdate
+     * @return array
+     */
     private function getCustomerEntityFieldsToUpdate(array $entitiesToUpdate): array
     {
         $firstCustomer = reset($entitiesToUpdate);