Embedded document changes are ignored if it was empty before

{
  "_id": "4c693f9f0f9d501c0d000000",
  "b": [
  ]
}

/** @Document(db="tests", collection="tests") */
class a
{
    /** @Id */
    protected $id;

    /** @EmbedOne(targetDocument="b", cascade="all") */
    protected $b;

    function getId()  {return $this->id;}
    function getB()   {return $this->b;}
    function setB($b) {$this->b = $b;}
}

/** @EmbeddedDocument */
class b
{
    /** @String */
    protected $val;
    function setVal($val) {$this->val = $val;}
    function getVal() {return $this->val;}
}

$a = $dm->loadByID('a', '4c693f9f0f9d501c0d000000');
$a->getB()->setVal('test');

$dm->persist($a);
$dm->flush();
$dm->getUnitOfWork()->clear();

$a = $dm->loadByID('a', $a->getId());
$c = ('test' === $a->getB()->getVal());

var_dump($c); // expecting "true", but outputs false