Hi, after upgrading to 2.9 (from 2.8) I now get an error when validating my schema

My setup is the following

Parent

/**
 * @ORM\Entity
 * @ORM\InheritanceType("SINGLE_TABLE")
 * @ORM\DiscriminatorColumn(name="discriminant", type="string")
 * @ORM\DiscriminatorMap({
 *     "type_child_a" = ChildA::class,
 *     "type_child_b" = ChildB::class,
 *     "other_type" = OtherType::class
 * })
 */
abstract class Parent

AbstractType

/**
 * @ORM\Entity
 */
abstract class AbstractType extends Parent

ChildA

/**
 * @ORM\Entity
 */
class ChildA extends AbstractType

ChildB

/**
 * @ORM\Entity
 */
class ChildB extends AbstractType

OtherType

/**
 * @ORM\Entity
 */
class OtherType extends Parent

My issue is that after upgrading to 2.9, when validating the schema I now get the error

Entity class 'AbstractType' is part of inheritance hierarchy, but is not mapped in the root entity 'Parent' discriminator map. All subclasses must be listed in the discriminator map.

If I'm not mistaken, doctrine shouldn't expect AbstractType to be in the DiscriminatorMap because it is an abstract class?

If it wasn't supposed to be working before, do you have an alternative to achieve my desired outcome?

Thank you!