#22 Reproduce issue in integration test

Model/Config/Parser/ParserChain.php

@@ -39,9 +39,11 @@ class ParserChain implements ConfigParserInterface
     public function execute(DOMElement $element): array
     {
         $data = [];
+
         foreach ($this->parser as $parser) {
-            $data = array_merge($data, $parser->execute($element));
+            $data[] = $parser->execute($element);
         }
-        return $data;
+
+        return array_merge(...$data);
     }
 }

Test/Integration/Model/Config/Parser/Query/FetchMediaFilesChainTest.php

+<?php
+declare(strict_types=1);
+
+namespace Firegento\ContentProvisioning\Test\Integration\Model\Config\Parser\Query;
+
+use Firegento\ContentProvisioning\Api\MediaFilesParserInterface;
+use Firegento\ContentProvisioning\Model\Config\Parser\Query\FetchMediaFilesChain;
+use PHPUnit\Framework\MockObject\MockObject;
+
+class FetchMediaFilesChainTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @var FetchMediaFilesChain
+     */
+    private $chain;
+
+    /**
+     * @var MediaFilesParserInterface|MockObject
+     */
+    private $parser1;
+
+    /**
+     * @var MediaFilesParserInterface|MockObject
+     */
+    private $parser2;
+
+    protected function setUp()
+    {
+        $this->parser1 = self::getMockBuilder(MediaFilesParserInterface::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        $this->parser2 = self::getMockBuilder(MediaFilesParserInterface::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+
+        $this->chain = \Magento\TestFramework\Helper\Bootstrap::getObjectManager()
+            ->create(FetchMediaFilesChain::class, ['parsers' => [
+                $this->parser1,
+                $this->parser2,
+            ]]);
+    }
+
+    public function testMergingData()
+    {
+        $this->parser1->method('execute')->willReturn([
+            'path/to/file1.png',
+            'path/to/file2.png',
+            'file3.png',
+        ]);
+        $this->parser2->method('execute')->willReturn([
+            'file3.png',
+            'some/other/path.jpg',
+        ]);
+
+        $result = $this->chain->execute('');
+
+        $this->assertSame([
+            'path/to/file1.png',
+            'path/to/file2.png',
+            'file3.png',
+            'file3.png',
+            'some/other/path.jpg',
+        ], $result);
+    }
+}