Why RestTemplate seems to increment response id?

I have created this integration test with RestTemplate:

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
@ExtendWith(SpringExtension::class)
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
internal class PatientControllerIntTest @Autowired constructor(
    private val patientRepository: PatientRepository,
    private val restTemplate: TestRestTemplate
) {
private val defaultPatientId = ObjectId.get()

    @LocalServerPort
    protected var port: Int = 0

    @BeforeEach
    fun setUp() {
        patientRepository.deleteAll()
    }

    private fun getRootUrl(): String = "http://localhost:$port/patients"
    private fun saveOnePatient() = patientRepository.save(Patient(defaultPatientId, "Name", "Description"))

    @Test
    fun `should return single patient by id`() {
        saveOnePatient()

        val response = restTemplate.getForEntity(
            getRootUrl() + "/$defaultPatientId",
            Patient::class.java
        )

        assertEquals(200, response.statusCode.value())
        assertNotNull(response.body)
        assertEquals(defaultPatientId, response.body?.id)
    }
}

I have run this application and it responds correctly.

I have debugged it at controller finding the correct id but when I get the response in the test case it is always the expected id incremented by one.