Issue with Document Manager

issue.php

<?php

// Controller
$tag = new Tag();
$dm->persist($tag);
$dm->flush();

$this->getContainer()->get('my_service')->import($content, $tag);

// Service
public function import(Content $content, Tag $tag)
{
    // With following line, it works.
    $tag = $this->contentRepository->getDocumentManager()->find(null, $tag->getId());

    // Otherwise:
    // A detached document was found through a reference during cascading a persist operation: my-tag (tag-id)

    $content->addTag($tag);
    $this->contentRepository->getDocumentManager()->flush();
}