Why simple query by id could cause timeout exception?

Occasionally, there are below exception in production environment,

2020-01-29 17:10:46.085 ERROR 2852 --- [o-8022-exec-258] c.c.p.common.dao.SearchDao               : Search person by id failed

java.net.SocketTimeoutException: 30,000 milliseconds timeout on connection http-outgoing-832 [ACTIVE]
        at org.elasticsearch.client.RestClient.extractAndWrapCause(RestClient.java:789) ~[elasticsearch-rest-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestClient.performRequest(RestClient.java:225) ~[elasticsearch-rest-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestClient.performRequest(RestClient.java:212) ~[elasticsearch-rest-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.internalPerformRequest(RestHighLevelClient.java:1433) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.performRequest(RestHighLevelClient.java:1403) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.performRequestAndParseEntity(RestHighLevelClient.java:1373) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.get(RestHighLevelClient.java:699) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]

Caused by: java.net.SocketTimeoutException: 30,000 milliseconds timeout on connection http-outgoing-832 [ACTIVE]
        at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.timeout(HttpAsyncRequestExecutor.java:387) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:92) ~[httpasyncclient-4.1.4.jar!/:4.1.4]
        at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:39) ~[httpasyncclient-4.1.4.jar!/:4.1.4]
        at org.apache.http.impl.nio.reactor.AbstractIODispatch.timeout(AbstractIODispatch.java:175) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.BaseIOReactor.sessionTimedOut(BaseIOReactor.java:263) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.AbstractIOReactor.timeoutCheck(AbstractIOReactor.java:492) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.BaseIOReactor.validate(BaseIOReactor.java:213) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.AbstractIOReactor.execute(AbstractIOReactor.java:280) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.BaseIOReactor.execute(BaseIOReactor.java:104) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.AbstractMultiworkerIOReactor$Worker.run(AbstractMultiworkerIOReactor.java:591) ~[httpcore-nio-4.4.11.jar!/:4.4.11]

but it's just a simple query, not a complicated query

 curl 'http://localhost:9201/person/_doc/30154410564?pretty'

and in this time, the load is very low

So why could exist these timeout exceptions? and there are many search query but why only this simple query by id could cause this exception?

The person index is synchronized from Oracle DB, and there is a scheduled task, every 10 min it will sync changed person to person index, and if access person index in this time, it will cause 30,000 milliseconds timeout. So how to resolve it? And it seems access by Java client, it will exist this phenomenon, but access by curl in command line it dose not exist this phenomenon.

PS:

health status index               uuid                   pri rep docs.count docs.deleted store.size pri.store.size
green  open   person              jb3msRw5S9ixgXN5SLd6bw   1   0  140754205     19239587     19.8gb         19.8gb

and in this time there was index write for the person index

RestClient Config:

private final RestHighLevelClient restHighLevelClient;
restHighLevelClient = new RestHighLevelClient(RestClient.builder(new HttpHost(host, port)));