Test simple REST with TJWSEmbeddedJaxrsServer + Mocktio: Not reproducible test result

I'm trying to test a simple rest API with a mocked service included. If I run the test several times in a row, there are two possible outcomes:

1. The test runs fine and everything is OK.
2. The test fails because the response is 204 - No Content. It looks like the mocked GWYServiceBean does not return an Object as intended. This leads to a wrong and emptpy response. I can't figure out where the problem is. Maybe one of you can help me out here.

The REST Service

@Inject
GWYServiceBean gwyServiceBean; // Should be mocked during the test

@POST
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON)
public GWYDevice addDevice(GWYDevice gwyDevice) {
    return gwyServiceBean.add(gwyDevice);
}

** Unit Test with Mockito/TJWSEmbeddedJaxrsServer **

@RunWith(MockitoJUnitRunner.class) public class GWYResourceTest {

@InjectMocks
private GWYResource sut = new GWYResource();
@Mock
private GWYServiceBean gwyServiceBeanMock;

private TJWSEmbeddedJaxrsServer server;
private ResteasyClient client;

@Before
public void setUp() {
    server = new TJWSEmbeddedJaxrsServer();
    server.getDeployment().getResources().add(sut);
    server.setPort(12345);
    server.start();
    client = new ResteasyClientBuilder().build();
}

@After
public void cleanUp() {
    client.close();
}

@Test
public void post() throws InterruptedException, ExecutionException {
    // Given
    String systemCode = "11111";
    String unitCode = "10000";
    long id = 1l;

    GWYDevice gwyDevice = new GWYDevice();
    gwyDevice.setSystemCode(systemCode);
    gwyDevice.setUnitCode(unitCode);

    GWYDevice persistedGwyDevice = new GWYDevice();
    persistedGwyDevice.setSystemCode(systemCode);
    persistedGwyDevice.setUnitCode(unitCode);
    persistedGwyDevice.setId(id);

    when(gwyServiceBeanMock.add(Mockito.any(GWYDevice.class))).thenReturn(persistedGwyDevice);

    // When
    Entity<GWYDevice> entity = Entity.json(gwyDevice);
    Response response = client.target("http://localhost:12345/gwy").request().post(entity);

    // Then
    assertTrue(response.getStatus() == OK.getStatusCode());
    GWYDevice returnedGwyDevice = response.readEntity(GWYDevice.class);
    assertNotNull(returnedGwyDevice.getId());
    assertEquals(gwyDevice.getSystemCode(), returnedGwyDevice.getSystemCode());
    assertEquals(gwyDevice.getUnitCode(), returnedGwyDevice.getUnitCode());
}