Doctrine referenceone gives only superclass

I’m refactoring old symfony 3 and doctrine 1.3 project;

I have 2 classes in same table: Journal and Conference mapped by their same parent Periodic.

/**
 * @ODMDocument(collection="periodics")
 * @ODMInheritanceType("SINGLE_COLLECTION")
 * @ODMDiscriminatorField("periodic_type")
 * @ODMDiscriminatorMap({
        "journal"=BundleDocumentJournal::class,
        "conference"=BundleDocumentConference::class,
    })
 * @GedmoSoftDeleteable(fieldName="deletedAt", timeAware=false)
 */
class Periodic {
...
}

/**
 * @ODMDocument()
 */
class Conference extends Periodic {
...
}

/**
 * @ODMDocument()
 */
class Journal extends Periodic {
...
}

I have also a class Assembly with referenceOne to Periodic:

class Assembly{
    /**
     * @ODMReferenceOne(
     *     targetDocument=Periodic::class,
     *     nullable=true
     * )
     */
    protected $periodic;

    /**
     * @return mixed
     */
    public function getPeriodic()
    {
        return $this->periodic;
    }

    /**
     * @param mixed $periodic
     * @return $this
     */
    public function setPeriodic($periodic)
    {
        $this->periodic = $periodic;
        return $this;
    }
}

The is a problem with getting Periodic from Assembly as I got object of Periodic class not Journal or Conference.
Please advice how to deal with it.