In case of inheritance, MongoRepository.saveAll(…) does not insert elements into the aggregate collection

open class Animal

class Bird(val color: String): Animal()

@Repository
interface AnimalRepository: MongoRepository<Animal, String>

@RunWith(SpringRunner::class)
@SpringBootTest
class DemoApplicationTests {

    @Autowired
    lateinit var animalRepository: AnimalRepository

    @Before
    fun cleanup() {
        animalRepository.deleteAll()
    }

    @Test
    fun testNormal() {

        animalRepository.save(Bird("green"))           // Saves to "Animal" collection as expected
        animalRepository.saveAll(listOf(Bird("red")))  // Saves to "Bird" collection even though it should go into the "Animal" collection

        val count = animalRepository.count() // Counts only the documents in the "Animal" collection

        assertEquals(2, count) // count == 1 
    }
}