Symfony 5 unittest can't fetch entity collection - phpunit

Question

my collection is empty while I try to access it in a unittest. In dev && prod env it is accessable as aspected.
One customerOrder can have multiple invoices and one invoice can have one customerOrder.
class CustomerOrderServiceTest extends KernelTestCase
{
use RefreshDatabaseTrait;
public function testAdd()
{
self::bootKernel();
$container = self::$container;
$orderRepository = $container->get(CustomerOrderRepository::class);
$customerOrder = $orderRepository->find(1);
foreach ($customerOrder->getInvoices() as $invoice) {
dump('Invoice id: ' . $invoice->getId());
}
dump('Invoice count: ' . $customerOrder->getInvoices()->count());
$invoiceRepository = $container->get(InvoiceRepository::class);
$invoice = $invoiceRepository->find(1);
dump('Order id: ' . $invoice->getCustomerOrder()->getId());
dump('Invoice count: ' . $invoice->getCustomerOrder()->getInvoices()->count());
}
}
The Output is
Testing Project Test Suite
^ "Invoice count: 0"
^ "Order id: 1"
^ "Invoice count: 0"
So the Order is accessable over the invoice but not the other way around.
The entities:
/**
* CustomerOrder
* #ORM\Entity(repositoryClass="App\Repository\CustomerOrderRepository")
*/
class CustomerOrder{
/**
* #var Collection
* #ORM\OneToMany(targetEntity="App\Entity\Invoice", mappedBy="customerOrder", cascade={"persist"}, fetch="EAGER")
*/
private $invoices;
}
/**
* Invoice
* #ORM\Entity(repositoryClass="App\Repository\InvoiceRepository")
* #ORM\Table(uniqueConstraints={#ORM\UniqueConstraint(name="number",columns={"number", "owner"})})
*/
class Invoice{
/**
* #var CustomerOrder
*/
private $customerOrder;
}
<?xml version="1.0" encoding="UTF-8"?>
<!-- https://phpunit.readthedocs.io/en/latest/configuration.html -->
<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="bin/.phpunit/phpunit.xsd"
colors="true"
bootstrap="tests/bootstrap.php"
>
<php>
<ini name="error_reporting" value="-1"/>
<server name="APP_ENV" value="test" force="true"/>
<server name="SHELL_VERBOSITY" value="-1"/>
<server name="SYMFONY_PHPUNIT_REMOVE" value=""/>
<server name="SYMFONY_PHPUNIT_VERSION" value="7.5"/>
<!-- ###+ symfony/framework-bundle ### -->
<env name="APP_ENV" value="test" force="true"/>
<env name="APP_SECRET" value="*****"/>
<!-- ###- symfony/framework-bundle ### -->
</php>
<testsuites>
<testsuite name="Project Test Suite">
<directory>tests</directory>
</testsuite>
</testsuites>
<filter>
<whitelist processUncoveredFilesFromWhitelist="true">
<directory suffix=".php">src</directory>
</whitelist>
</filter>
<listeners>
<listener class="Symfony\Bridge\PhpUnit\SymfonyTestsListener" />
</listeners>
</phpunit>