SpringBoot Socket is closed abnormally, is this a bug?

Spring boot release 2.2.4

when I write a controller:

@RestController
public class Hello {

    @GetMapping("list")
    public List<String> list() {
        List list = new ArrayList();
        list.add("lxm");

        return list;
    }

    @GetMapping("hello")
    public String hello() {
        return "lxm";
    }
}

get "localhost:8080/list" twice in a clinet which set keepalive=true, the socket is closed, and I get the log:

2020-08-11 11:11:12.966 DEBUG 7136 --- [nio-8080-exec-7] o.apache.coyote.http11.Http11Processor   : Socket: [org.apache.tomcat.util.net.NioEndpoint$NioSocketWrapper@1356d886:org.apache.tomcat.util.net.NioChannel@7765be10:java.nio.channels.SocketChannel[connected local=/10.6.227.126:8080 remote=/10.6.136.48:7768]], Status in: [OPEN_READ], State out: [CLOSED]
2020-08-11 11:11:12.966 DEBUG 7136 --- [nio-8080-exec-7] o.apache.tomcat.util.threads.LimitLatch  : Counting down[http-nio-8080-exec-7] latch=1

however when get "localhost:8080/hello" twice in a clinet which set keepalive=true, the socket is keep open, and the log:

2020-08-11 11:16:44.752 DEBUG 7136 --- [nio-8080-exec-2] m.m.a.RequestResponseBodyMethodProcessor : Writing ["lxm"]
2020-08-11 11:16:44.753 DEBUG 7136 --- [nio-8080-exec-2] o.s.web.servlet.DispatcherServlet        : Completed 200 OK
2020-08-11 11:16:44.753 DEBUG 7136 --- [nio-8080-exec-2] o.a.tomcat.util.net.SocketWrapperBase    : Socket: [org.apache.tomcat.util.net.NioEndpoint$NioSocketWrapper@a74259:org.apache.tomcat.util.net.NioChannel@7765be10:java.nio.channels.SocketChannel[connected local=/10.6.227.126:8080 remote=/10.6.136.48:10024]], Read from buffer: [0]
2020-08-11 11:16:44.754 DEBUG 7136 --- [nio-8080-exec-2] org.apache.tomcat.util.net.NioEndpoint   : Socket: [org.apache.tomcat.util.net.NioEndpoint$NioSocketWrapper@a74259:org.apache.tomcat.util.net.NioChannel@7765be10:java.nio.channels.SocketChannel[connected local=/10.6.227.126:8080 remote=/10.6.136.48:10024]], Read direct from socket: [0]
2020-08-11 11:16:44.754 DEBUG 7136 --- [nio-8080-exec-2] o.apache.coyote.http11.Http11Processor   : Socket: [org.apache.tomcat.util.net.NioEndpoint$NioSocketWrapper@a74259:org.apache.tomcat.util.net.NioChannel@7765be10:java.nio.channels.SocketChannel[connected local=/10.6.227.126:8080 remote=/10.6.136.48:10024]], Status in: [OPEN_READ], State out: [OPEN]

that is strange.