Spring Boot elasticsearch produce warnings: Transport response handler not found of id - spring-boot

Question

I setuped a web app with org.springframework.boot:spring-boot-starter-data-elasticsearch. Everything work well - I can populate indexes with my stand alone Elasticsearch 5. But I continue to receive some weird warnings:
2018-05-08 03:07:57.940 WARN 32053 --- [ient_boss][T#7]] o.e.transport.TransportService : Transport response handler not found of id [5]
2018-05-08 03:08:02.949 WARN 32053 --- [ient_boss][T#8]] o.e.transport.TransportService : Transport response handler not found of id [7]
2018-05-08 03:08:07.958 WARN 32053 --- [ient_boss][T#1]] o.e.transport.TransportService : Transport response handler not found of id [9]
2018-05-08 03:08:12.970 WARN 32053 --- [ient_boss][T#2]] o.e.transport.TransportService : Transport response handler not found of id [11]
...
Simple app to reproduce:
#SpringBootApplication
public class App {
#Configuration
#EnableElasticsearchRepositories(basePackages = "com.test")
public class EsConfig {
#Value("${elasticsearch.host}")
private String esHost;
#Value("${elasticsearch.port}")
private int esPort;
#Value("${elasticsearch.clustername}")
private String esClusterName;
#Bean
public TransportClient client() throws Exception {
Settings esSettings = Settings.builder().put("cluster.name", esClusterName).build();
InetSocketTransportAddress socketAddress = new InetSocketTransportAddress(
InetAddress.getByName(esHost), esPort);
return new PreBuiltTransportClient(esSettings).addTransportAddress(socketAddress);
}
#Bean
public ElasticsearchOperations elasticsearchTemplate(Client client) throws Exception {
return new ElasticsearchTemplate(client);
}
}
public static void main(String[] args) {
SpringApplication.run(App.class, args);
}
}
My compose file for ES
version: "2.3"
services:
elasticsearch:
image: 'docker.elastic.co/elasticsearch/elasticsearch:5.6.8
ports:
- "9200:9200"
- "9300:9300"
environment:
- xpack.security.enabled=false
- ES_JAVA_OPTS=-Xms700m -Xmx700m
- PATH_LOGS="/tmp/el-log"
- cluster.name=dou
cpu_shares: 1024
mem_limit: 1024MB
As a project transitive dependency I have org.elasticsearch.client:transport:5.6.8. Looks like versions of ES instance & library are the same.
So, what does this warning mean & how should we deal with it?