Transaction not working when using multiple repository

Question

I try to save 2 Documents in the same transaction. But when an exception is thrown just after the first save(), the first document is still saved. as if there was no transaction at all.

I use MongoDB Atlas, version 6.0.11

here my service :

@Service
@RequiredArgsConstructor
public class ApiService {

    private final FirstRepository firstRepository;
    private final SecondRepository secondRepository;

    @Transactional
    public Mono<SecondDocument> save(FirstDocument firstDocument, SecondDocument secondDocument) {
        firstRepository.save().subscribe();
        if(true) {
            throw new RuntimeException("test");
        }
        return secondRepository.save(secondDocument);
    }
}

and my mongo's configuration class:

@Configuration
@EnableTransactionManagement
public class MongoReactiveConfig extends AbstractReactiveMongoConfiguration {

    @Bean
    ReactiveMongoTransactionManager transactionManager(ReactiveMongoDatabaseFactory factory) {
        return new ReactiveMongoTransactionManager(factory);
    }

    @Override
    public MongoClient reactiveMongoClient() {
        return MongoClients.create("mongodb+srv://user:password@url/db&replicaSet=replicaSet");
    }

    @Override
    protected String getDatabaseName() {
        return "database";
    }
}

and my spring boot app :

@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class})
public class ApiApplication {

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

Can anyone help me understand why the firstDocument is still saved please ?