How to configure extbase to fetch inline / IRRE relations to tt_content? - typo3

Question

I've built a extension where I can assign content elements (tt_content) inline (IRRE) to my entries in the TYPO3 Backend.
But I can't figure out how I can resolve the relations so that I can display the assigned content elements in the frontend.
I've created the extension with the 'Extension Builder' in TYPO3 9.5.5.
I created a Model Object 'Content' and selected 'Map to existing table' in the 'Domain object settings' and choosed 'tt_content'.
In my model 'ProductTab' I configured a 1:n relation of type 'Inline (IRRE)' with the name 'content' to my 'Content' model.
If I add content elements to my model entries in the Backend, it works like expected. The relational fields in the DB are mapped correctly. With the 'Extbase Variable Dump' I can see that all my relations are passed to the front end plugin, but my 'content' relation is an empty 'ObjectStorage'.
What am I missing so that the extbase persistence fetches my content relations?
ext_tables.sql excerpts
CREATE TABLE tx_myext_domain_model_producttab (
product int(11) unsigned DEFAULT '0' NOT NULL,
title varchar(255) DEFAULT '' NOT NULL,
content int(11) unsigned DEFAULT '0' NOT NULL,
);
CREATE TABLE tt_content (
producttab int(11) unsigned DEFAULT '0' NOT NULL,
);
TCA excerpts
'content' => [
'exclude' => false,
'label' => 'producttab.content',
'config' => [
'type' => 'inline',
'foreign_table' => 'tt_content',
'foreign_field' => 'producttab',
'maxitems' => 9999,
'appearance' => [
'collapseAll' => 0,
'levelLinksPosition' => 'top',
'showSynchronizationLink' => 1,
'showPossibleLocalizationRecords' => 1,
'showAllLocalizationLink' => 1
],
],
],
Since I can add and edit the content elements in the Backend, I think the DB and TCA relation is fine.
But something is still missing, so that the relation is fetched and provided to the FE template so that I can display the assigned content elements with a ViewHelper.
ProductTab.php excerpts
class ProductTab extends \TYPO3\CMS\Extbase\DomainObject\AbstractValueObject
{
protected $content = null;
protected function initStorageObjects()
{
$this->content = new \TYPO3\CMS\Extbase\Persistence\ObjectStorage();
}
public function addContent(\MyExt\Domain\Model\Content $content)
{
$this->content->attach($content);
}
public function removeContent(\MyExt\Domain\Model\Content $contentToRemove)
{
$this->content->detach($contentToRemove);
}
public function getContent()
{
return $this->content;
}
public function setContent(\TYPO3\CMS\Extbase\Persistence\ObjectStorage $content)
{
$this->content = $content;
}
}
Extbase Variable Dump
Screenshot of debug output
On the screenshot you can see the fetched products (one item in this case) and the producttabs (also 1 item). There is one content element assigned, but the property content of the ProductTab with uid=1 is empty.
Update
Since I'm still not able to solve this issue, I've created a minimalistic extension with the Extension Builder to demonstrate the issue: https://github.com/apiening/demo_irrecontent
The extension is the most simple one I could create with one model holding only one property and the relation to tt_content. It has a plugin list which also shows the debug output.
I've put some more details in the README.mdof the GitHub project.