MongoDbTest in Spring Boot (memory) not honoring unique index

Question

I'm doing a battery tests in a Spring Boot applications, using in memory MongoDbTest, and my test for testing unique/non-duplicate indexes is failing (it lets me create records with duplicate keys, and the relevant test fail).

For declaring the unique index, I'm using the annotations @Indexed(unique=true)

This is my code

Entity class

@Document(collection="products")
public class ProductEntity {

    // Id internal (db managed id)
    @Id
    private String id;

    // Used for manage optimistic locking
    @Version
    private Integer version;

    // Create unique index (external id)
    @Indexed(unique = true)
    private int productId;

    private String name;
    private int weight;

    public ProductEntity(int productId, String name, int weight) {
        this.productId = productId;
        this.name = name;
        this.weight = weight;
    }
}

Test class

@DataMongoTest
public class PersistenceTests {

    @Autowired
    private ProductRepository repository;

    private ProductEntity savedEntity;

    @BeforeEach
    public void setupDb() {
        repository.deleteAll();

        ProductEntity entity = new ProductEntity(1, "n", 1);
        savedEntity = repository.save(entity);

        assertEqualsProduct(entity, savedEntity);
    }
    @Test
    public void duplicateError() {
        ProductEntity entity = new ProductEntity(savedEntity.getProductId(), "n", 1);
        Assertions.assertThrows(DuplicateKeyException.class, () -> {
            repository.save(entity);
        });
    }
}

Repository

public interface ProductRepository extends PagingAndSortingRepository<ProductEntity, String> {
    Optional<ProductEntity> findByProductId(int productId);
}

The embed MongoDB is declared as dependency in my build.gradle as

// Embedded MongoDB for test only
testImplementation('de.flapdoodle.embed:de.flapdoodle.embed.mongo')