WSO2 Identity Server userAdmin Web Service Failed to Re-use - wso2

Question

My WSO2 Identity Server version is 4.0.0 final. I have turned on admin service.
Here are the two requests.
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:mgt="http://mgt.user.carbon.wso2.org">
<soapenv:Header/>
<soapenv:Body>
<mgt:listUsers>
<!--Optional:-->
<mgt:filter>*</mgt:filter>
</mgt:listUsers>
</soapenv:Body>
</soapenv:Envelope>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:mgt="http://mgt.user.carbon.wso2.org">
<soapenv:Header/>
<soapenv:Body>
<mgt:getAllRolesNames/>
</soapenv:Body>
</soapenv:Envelope>
If i call them separately in Test Case, it is okay. If i call listUsers first, and then getAllRolesNames (In SOAPUI, call listUser first, then change the Envelope to getAllRolesName one), it shows
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
<soapenv:Body>
<soapenv:Fault>
<faultcode>soapenv:Server</faultcode>
<faultstring>unknown</faultstring>
<detail>
<ns:UserAdminUserAdminException xmlns:ns="http://mgt.user.carbon.wso2.org">
<UserAdminException xsi:type="ax2566:UserAdminException" xmlns="http://mgt.user.carbon.wso2.org" xmlns:ax2566="http://common.mgt.user.carbon.wso2.org/xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<ax2566:errorMessage xsi:nil="true"/>
</UserAdminException>
</ns:UserAdminUserAdminException>
</detail>
</soapenv:Fault>
</soapenv:Body>
</soapenv:Envelope>
Does it mean there are some bug inside server side implementation ?
I tested them in SOAPUI. The login session is not expired, i can still use it for querying. Does it mean i have to create ws client bean using request scope instead of session scope?
Thanks a lot.