Symfony 5 tests phpunit Doctrine problem update database row

I have the following problem with doctrine when testing a symfony 5 application. Instead of updating the rows in the database, new rows are created when the persist () method is called or when cascade = {“persist”} is defined in Entity. The above issue only occurs in a test environment. Everything works fine in the app itself.
sample test code (maximally simplified to show the problem)

class GetReadyArticlesTest extends FunctionalDBTest
{
    protected function setUp():void
    {
        parent::setUp();
        $this->addFixture(new ConfigurationFixtures());
        $this->addFixture(new CopyWriterTextOrderFixtures());
        $this->executeFixtures();

    }
    protected function tearDown(): void
    {
        parent::tearDown();
    }
    public function testProcessSaveArticles()
    {
        $textOrderRepository = $this->entityManager->getRepository(CopywriterTextOrder::class);
        $textOrderEntity = $textOrderRepository->find(1);
        $textOrderEntity->setCharacters(4500);
        $this->entityManager->persist($textOrderEntity);
        $this->entityManager->flush();
    }
}

Abstract class FunctionalDBTest:

abstract class FunctionalDBTest extends FunctionalTest
{
    /**
     * @var EntityManagerInterface
     */
    protected $entityManager;

    /**
     * @var ORMExecutor
     */
    private $fixtureExecutor;

    /**
     * @var ContainerAwareLoader
     */
    private $fixtureLoader;

    protected function setUp(): void
    {
        parent::setUp();

        if ($this->getContainer()->getParameter('kernel.environment') !== 'test') {
            die('Wymagane środowisko testowe');
        }

        $this->entityManager = $this
            ->getContainer()
            ->get('doctrine')
            ->getManager();
        $schemaTool = new SchemaTool($this->entityManager);
        $schemaTool->updateSchema($this->entityManager->getMetadataFactory()->getAllMetadata());
    }

    protected function addFixture(FixtureInterface $fixture): void
    {
        $this->getFixtureLoader()->addFixture($fixture);
    }


    protected function executeFixtures(): void
    {
        $this->getFixtureExecutor()->execute($this->getFixtureLoader()->getFixtures());
    }

    private function getFixtureExecutor(): ORMExecutor
    {
        if (!$this->fixtureExecutor) {
            $this->fixtureExecutor = new ORMExecutor($this->entityManager, new ORMPurger($this->entityManager));
        }

        return $this->fixtureExecutor;
    }

    private function getFixtureLoader(): ContainerAwareLoader
    {
        if (!$this->fixtureLoader) {
            $this->fixtureLoader = new ContainerAwareLoader($this->getContainer());
        }

        return $this->fixtureLoader;
    }

    protected function tearDown(): void
    {
        (new SchemaTool($this->entityManager))->dropDatabase();

        parent::tearDown();

        $this->entityManager->close();
        $this->entityManager = null; 
    }
}

Removing the persist () method in this example fixes the problem. But in case I want to save a new relation to the table, it also generates a new main object. the problem only occurs in tests.