Spring sleuth is exporting spans under different trace ids for a same request trace

Question

My spring boot service is giving different trace ids for external API calls for a same request trace. Expectation is these external http API requests are must be under same request trace id.

Please assist on this. I have create a sample reproducible code, here are the details:

Main class

@SpringBootApplication
public class RxjavaAndSpringSleuthApplication {

    public static void main(String[] args) {
        SpringApplication.run(RxjavaAndSpringSleuthApplication.class, args);
    }

    @Bean
    public ExecutorService appExecutorService() {
        return Executors.newCachedThreadPool();
    }

    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

Service class

@Service
public class TestService {
    @Autowired
    private RestTemplate restTemplate;

    @Autowired
    private ExecutorService appExecutorService;

    public Object getOutput() {
        Single<Object> externalIOBoundApiCallOne = Single.fromSupplier(
                        () -> {
                            Future<Object> obj =
                                    appExecutorService.submit(() -> {
                                        return restTemplate.getForObject("https://random-data-api.com/api/address/random_address", Object.class);
                                    });
                            return obj.get();
                        }
                )
                .timeout(5000, TimeUnit.MILLISECONDS)
                .subscribeOn(Schedulers.io())
                .onErrorReturn(t -> appExecutorService.submit(() -> "ERROR"));

        Single<Object> externalIOBoundApiCallTwo = Single.fromSupplier(
                        () -> {
                            Future<Object> obj =
                                    appExecutorService.submit(() -> {
                                        return restTemplate.getForObject("https://random-data-api.com/api/app/random_app", Object.class);
                                    });
                            return obj.get();
                        }
                )
                .timeout(5000, TimeUnit.MILLISECONDS)
                .subscribeOn(Schedulers.io())
                .onErrorReturn(t -> appExecutorService.submit(() -> "ERROR"));

        Object result = Single.zip(externalIOBoundApiCallOne, externalIOBoundApiCallTwo,
                        (externalIOBoundApiCallOneResponse, externalIOBoundApiCallTwoResponse) -> externalIOBoundApiCallOneResponse)
                .blockingGet();
        System.out.println(result);
        return result;
    }
}

Controller class

@RestController
public class TestController {
    private TestService service;
    public TestController(TestService service) {
        this.service = service;
    }

    @GetMapping("/get1")
    public Object getRandomData() {
        return service.getOutput();
    }
}

application.properties

server.port=8080
spring.application.name=rxjava-and-spring-sleuth

spring.zipkin.baseUrl=http://localhost:9411/
spring.sleuth.sampler.probability=1.0

loggingpattern.level="%5p [rxjava-and-spring-sleuth, traceId=%X{X-B3-TraceId:-}, spanId=%X{X-B3-SpanId:-}, %X{X-Span-Export:-}, %X{clientName}]"

dependencies used

plugins {
    id 'java'
    id 'org.springframework.boot' version '2.6.0'
    id 'io.spring.dependency-management' version '1.0.9.RELEASE'
}

group = 'com.example'
version = '0.0.1-SNAPSHOT'

/*java {
    sourceCompatibility = '1.8'
}*/

repositories {
    mavenCentral()
}

ext {
    set('springCloudVersion', "2021.0.2")
}

dependencyManagement {
    imports {
        mavenBom "org.springframework.cloud:spring-cloud-dependencies:${springCloudVersion}"
    }
}

dependencies {
    implementation 'io.reactivex.rxjava3:rxjava:3.0.0'
    implementation 'org.springframework.boot:spring-boot-starter-actuator'
    implementation 'org.springframework.boot:spring-boot-starter-web'
    implementation 'org.springframework.cloud:spring-cloud-starter-sleuth'
    implementation 'org.springframework.cloud:spring-cloud-sleuth-zipkin'

    testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

Traces displayed in Grafana Tempo dashboard:

I'm struggling to solve this issue. Can someone please assist on this? thanks in advance!