To reproduce the issue:
1. wsgen -cp . wstest.EchoImpl against the following java file:
package wstest;

import javax.jws.WebMethod;
import javax.jws.WebParam;
import javax.jws.WebService;
import javax.xml.ws.Holder;
import java.rmi.RemoteException;
import javax.jws.soap.SOAPBinding;

@WebService
@SOAPBinding(style=SOAPBinding.Style.RPC)
public class EchoImpl {
    @WebMethod
    public String holderOperation(@WebParam(name="holder1", mode=WebParam.Mode.INOUT)Holder<String> holder1,
                                  @WebParam(name="holder2", mode=WebParam.Mode.INOUT)Holder<String> holder2) throws RemoteException{
        holder1.value += "1";
        holder2.value += "2";
        return "success";
    }
}

2. start the web service with the following java file: 
package wstest;

import javax.xml.ws.Endpoint;

public class EchoService {
    public static void main(String[] args) {
        Endpoint.publish("http://localhost:8080/WebServices/echoimpl", new EchoImpl());
    }
}

3. generate the client:
wsimport -keep -p wstest.server http://localhost:8080/WebServices/echoimpl?wsdl

4. run the client test with the following java file:
import java.rmi.*;
import javax.naming.*;
import wstest.server.*;
import javax.xml.ws.Holder;

public class EchoClient {
    public static void main(String[] args) {
        EchoImplService service = new EchoImplService();
        EchoImpl port = service.getPort(EchoImpl.class);
        Holder h1 = new Holder("1");
        Holder h2 = new Holder("2");
        String result = port.holderOperation(h1, h2);
        
        System.out.println(result);
        System.out.println(h1.value);
        System.out.println(h2.value);
    }
}

Will get:
null
11
22

Note:
1. Obviously some changes made the result better, now only the return value is null.
2. If removing @SOAPBinding(style=SOAPBinding.Style.RPC) from EchoImpl java file or inserting a non-holder parameter at the first place in holderOperation method, it will have return value.