Single Table Inheritance - Columns of one hierarchy are dropped

/**
 * @ORM\Entity
 * @ORM\Table(name = "a")
 * @ORM\ChangeTrackingPolicy("DEFERRED_EXPLICIT")
 * @ORM\InheritanceType("SINGLE_TABLE")
 * @ORM\DiscriminatorColumn(name="type", type="string", length=20)
 * @ORM\DiscriminatorMap({
 *     "c"   = "C",
 *     "d"   = "D",
 *     "e"   = "E"
 * })
 */
abstract class A { }

/**
 * @ORM\Entity
 */
abstract class B extends A { }

/**
 * @ORM\Entity
 */
class C extends B { }

/**
 * @ORM\Entity
 */
class D extends B { }

/**
 * @ORM\Entity
 */
class E extends A { }