ENGCOM-2302: [Forwardport] Fix for GitHub issue #14035. #16742

 - Merge Pull Request #16742 from eduard13/magento2:2.3-develop-port-14048
 - Merged commits:
   1. 9fb7323
   2. 69c9a63
   3. 1128446

Author: magento-engcom-team

app/code/Magento/Catalog/Model/Api/SearchCriteria/CollectionProcessor/FilterProcessor/ProductCategoryFilter.php

@@ -21,8 +21,13 @@ class ProductCategoryFilter implements CustomFilterInterface
      */
     public function apply(Filter $filter, AbstractDb $collection)
     {
-        $conditionType = $filter->getConditionType() ?: 'eq';
-        $categoryFilter = [$conditionType => [$filter->getValue()]];
+        $value = $filter->getValue();
+        $conditionType = $filter->getConditionType() ?: 'in';
+        $filterValue = [$value];
+        if (($conditionType === 'in' || $conditionType === 'nin') && is_string($value)) {
+            $filterValue = explode(',', $value);
+        }
+        $categoryFilter = [$conditionType => $filterValue];
 
         /** @var Collection $collection */
         $collection->addCategoriesFilter($categoryFilter);

app/code/Magento/Catalog/Test/Unit/Model/Api/SearchCriteria/CollectionProcessor/FilterProcessor/ProductCategoryFilterTest.php

@@ -66,7 +66,7 @@ public function testApplyWithoutCondition()
 
         $collectionMock->expects($this->once())
             ->method('addCategoriesFilter')
-            ->with(['eq' => ['value']]);
+            ->with(['in' => ['value']]);
 
         $this->assertTrue($this->model->apply($filterMock, $collectionMock));
     }