Asynchronous API get blocked after processing N requests

Question

I developed a simple asynchronous API in Scala using Spring MVC. In the Controller I am using DeferredResult<Map<String,Integer>> response = new DeferredResult<>(360000L,new String("PROCESSING_TIMEOUT")); in order to return the message PROCESSING_TIMEOUT in case of excessivly long processing time. Then, in order to test this API, I send 100 simultaneous requests. The problem is that after a successful processing of around 50 first requests, the program gets blocked and waits 360000 milliseconds, then returns the message PROCESSING_TIMEOUT, and continues processing the rest of requests. Then it gets blocked again and unblocks in 360000 milliseconds. I don't understand such behaviour. Why does it happen? And how to resolve it? I tried to play with the core pool size and maximum pool size parameters, but it didn't solve the problem.

@Configuration
@SpringBootApplication
@EnableAsync
public class AppConfig implements AsyncConfigurer {
    @Bean
    public javax.validation.Validator localValidatorFactoryBean() {
        return new LocalValidatorFactoryBean();
    }

    @Override
    public Executor getAsyncExecutor() {
        //return new SimpleAsyncTaskExecutor();
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(2);
        executor.setMaxPoolSize(2);
        executor.setQueueCapacity(1000);
        executor.setThreadNamePrefix("MyExecutor-");
        executor.initialize();
        return executor;
    }

    @Override
    public AsyncUncaughtExceptionHandler getAsyncUncaughtExceptionHandler() {
        return new MyUncaughtExceptionHandler();
    }

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

}