primeFaces 5.2 selectOneMenu valueChangeListener why event get itemLabel instead of itemValue?

Question

I got a very funny issues: When I tried to access selectOneMenu selected value by using valueChangeListener's valueChangeEvent. I got itemLabel instead of itemValue.

jsf snappet:

<p:selectOneMenu id="position" value="#{employmentBean.currentEmployment.positionId}" required="true" effect="fold" editable="true"
                 requiredMessage="Please select an available option." hideNoSelectionOption="true" immediate="true" valueChangeListener="#{employmentControlBean.handlepositionIdSelected}">
   <p:ajax update="@form" />
   <f:selectItem itemLabel="Select Position to Create Employment." itemValue="" />
   <f:selectItems value="#{positionBean.vacantPositionMap.entrySet()}" var="entry" itemLabel="#{entry.value}" itemValue="#{entry.key}"/>
</p:selectOneMenu>

my vacantPositionMap is a

Map<String,String> 

style key/value pair, key is position Id, value is position Name.

my backing bean:

@Named(value = "employmentControlBean")
@SessionScoped
public class EmploymentControlBean(){
    public void handlepositionIdSelected(ValueChangeEvent event){
    ........
    String positionId = (String) event.getNewValue();
    positionBean.setCurrentPosition((Position) positionBean.getPositionCache().get(positionId));
    employmentBean.getCurrentEmployment().setPositionId(positionId);
  }
}

from above snappet you can see. I am trying to fetch selectOneMenu selected value on valueChangeEvent.

Backing bean method by using ValueChangeEvent, I suppose JSF's itemValue will be passed to backing bean by using ValueChangeEvent.

This is what I found in my html by using firebug:

From above image you can see, primeFaces rendering html correctly, but when I tried to access selected value I get "President". It is not I expected, I expect will get "POSTN20161215xxxxxx".

My question is: Why ValueChangeEvent pass itemLabel instead of itemValue? How I can get the correct value?

Please advise!!