Child-view gets rendered twice - forward

Question

I've noticed that my child-view gets rendered twice for some reasons.
Here is a simple test-case:
Let's assume I've two controllers, ControllerA and ControllerB. I'm forwarding the action forwardTestAction from ControllerA to ControllerB and display the view returned from ControllerB as a child-view.
ControllerA::forwardTestAction()
public function forwardTestAction()
{
$childView = $this->forward()->dispatch(ControllerB::class, [
'action' => 'forward-test',
]);
$mainView = new ViewModel();
$mainView->addChild($childView, 'content');
return $mainView;
}
ControllerB::forwardTestAction()
public function forwardTestAction()
{
$number = new \stdClass();
$number->a = 10.4;
$view = new ViewModel([
'number' => $number,
]);
return $view;
}
Template forward-test.phtml of ControllerA
<?= $content; ?>
Template forward-test.phtml of ControllerB
<?php
$number->a = $this->currencyFormat($number->a);
echo $number->a;
The currencyFormat plugin throws an exception, because number gets formatted twice. The reason is because the child-view itself gets rendered twice.
That's a really weird behaviour and causes not only performance issues but also many other problems e.g. numberFormatting etc.
I'm doing something wrong?
Also I should mention that the html only gets outputted once, but the template gets rendered twice. Maybe it's just a weird constellation of my current setup. I also posted an issue on Github and they were not able to reproduce it.