I simplified my problem to this pseudo code.
My controller

@RestController
@RequestMapping("api")
@RequiredArgsConstructor
public class TestController {
  private final Service service;
  @PostMapping("/test")
  public Mono<Void> test(@RequestBody @Valid Flux<TestDto> testDtoFlux) {
    return testDtoFlux
      .map(service::test)
      .then();
  }

test:

  void setup() {
    service = Mock(Service)
    controller = new TestController(service)
  }

  def "Should test abc"() {
    setup:
        def testDto = new TestDto(11L, true)

    when:
        def result = StepVerifier.create(controller.test(Flux.just(testDto)))

    then:
        result
          .verifyComplete()

        1 * service.test(_) >> true
  }

test everytime says that service.test is never executed. but when I changed code to something like this test passed:

  public Mono<Void> test(@RequestBody @Valid Flux<TestDto> testDtoFlux) {
    testDtoFlux
      .map(service::test)
      .then().block();
    return Mono.empty();
  }

I’m not sure why this is happening. I was trying to fix that with for example that:

result
  .expectSubscription()
  .thenAwait(Duration.ofSeconds(5))
  .expectComplete()
  .verify();

but result is always the same. Maybe you can see what I’m doing wrong?

ps. When I run the application, everything works fine. I only have a problem with this test.