Nested embedded object incorrectly hydrates to null (Doctrine2, php)

Question

Summary: Doctrine Embedded types are failing to be populated when they are contained in entities which are themselves contained in another top-level entity.

Details: I have a top-level entity A which has a ManyToOne relationship with an intermediate-level entity B, which in turn has an Embedded object C.

When I retrieve the top-level entity (using $entityManager->getRepository(A::class)->find($aid) ), the returned A object's $b variable has its $c property explicitly set to null (rather than - as you might expect - being absent altogether). Querying the $c property does not cause B to hydrate - instead $c remains null.

If I first put a dummy call into to get a simple property of B e.g. reference, then B hydrates correctly and $c is populated correctly.

Similarly, if I come in at the intermediate level with $entityManager->getRepository(B::class)->find($bid) then there is no problem either.

Is this a limitation in Doctrine?

(A possible complicating factor is the fact that A itself is a base class for 2 further entities)

/**
 * @Entity
 * @InheritanceType("SINGLE_TABLE")
 * @DiscriminatorColumn(name="type", type="string")
 * @DiscriminatorMap({"child1" = "Child1 "child2" = "Child2"})
 * @Table(name="A")
 */
class A
{
    /**
     * @ManyToOne(targetEntity="Model\B", inversedBy="As")
     * @JoinColumn(name="b_id", referencedColumnName="id",)
     * @var B
     */
    public $b;
}

/**
 * @Entity
 * @Table(name="b")
 */
class B
{
    /**
     * @Embedded(class="Model\C", columnPrefix = false)
     * @var C
     */
    public $c;

    /**
     * @Column(type="string")
     * @var string
     */
    public $reference;
}

/**
 * @Embeddable
 */
class C
{
    /**
     * @Column(type="smallint")
     * @var int
     */
    public $type;   
}