PrimeFaces splitButton: immediate="true" does not seem to work - jsf

Question

Sidenote: It works with Command-Buttons.
What I Have:
A simple form where name and address from a customer are entered. In a data-table below, each row contained (in the working version) the following fields:
Name of the customer
A Command-Button which redirects to another page and shows the customers orders (This worked using <p: commandButton immediate="true">)
Another Command-Button which displayed the past orders.
Another Command-Button which was responsible for updating customer-data.
Since I didn't want to have three Command-Buttons in each row I decided to use a Split-Button.
Problem:
<p:splitButton immediate="true">
The form asks me to fill the missing data (name and address) which are set to required="true.
Question:
As far as I understand the attribute immediate="true is used to overcome this issue. So what am I missing ?
Code:
<h:form>
<p:growl id="growl" sticky="false" life="3500" showDetail="false"/>
<h:panelGrid id="customer_grid" columns="2" cellspacing="1" cellpadding="5">
<h:outputLabel id="label" for="name" value="Kunde:" style="font-weight:bold"/>
<p:inputText id="name" value="#{customerController.customer.name}" required="true" requiredMessage="Name eingeben!"/>
<h:outputLabel for="address" value="Adresse:" style="font-weight: bold" />
<p:inputText id="address" value ="#{customerController.customer.address}" required="true" requiredMessage="Adresse eingeben!"/>
<p:commandButton action = "#{customerController.createCustomer}" value="Speichern" style="margin-right:10px"
actionListener="#{growlController.saveMessage}" ajax="true"
onclick="PF('blockUIWidget').block()" oncomplete="PF('blockUIWidget').unblock()" styleClass="save-button"/>
<p:commandButton onclick="history.back(); return false;" value="Abbrechen" ajax="true"/>
<pe:blockUI widgetVar="blockUIWidget">
<h:panelGrid columns="2">
<p:graphicImage id="loader" name="images/ajax-loader.gif" style="margin-right: 12px; vertical-align: middle;" rendered="true"/>
<h:outputText value="Please wait..." style="white-space: nowrap;"/>
</h:panelGrid>
</pe:blockUI>
</h:panelGrid>
<br/>
<br/>
<p:dataTable var="customer" value="#{customerController.allCustomers}" resizableColumns="true" tableStyle="width: auto"
rendered="#{not empty customerController.allCustomers}">
<p:column headerText="customer" style="width: 300px">
<h:outputText value="#{customer.name}" />
</p:column>
<p:column>
<p:splitButton value="current order" action="#{userController.setup(customer, 'lastOrder')}" immediate="true">
<p:menuitem value="old orders" action="#{userController.setup(customer, 'oldOrders')}" immediate="true"/>
<p:menuitem value="edit" action="#{userController.setup(customer, 'update')}" immediate="true"/>
</p:splitButton>
</p:column>
</p:dataTable>
EDIT:
According to the comment of BalusC I've put the two parts in separate forms.
Effect: The message to fill out the above form does not show up, but the redirect is not happening either.
EDIT2:
The purpose of the method userController.setup(customer, 'String') is basically to inject the customer who is represented for each row. The String is returned for redirecting purposes which are set in the faces-config.xml and as I said: It works when I use Command-Buttons instead.
CODE:
#Named
#SessionScoped
public class UserController implements Serializable{
#Inject
private Customer customer;
#EJB
private CustomerService customerService;
public UserController(){
}
public List<Item> getItems(){
return customerService.getItems(customer);
}
public Ordery getCurrentOrder(){
return customerService.getCurrentOrder(customer);
}
public Customer getCustomer() {
return customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}
public CustomerService getCustomerService() {
return customerService;
}
public void setCustomerService(CustomerService customerService) {
this.customerService = customerService;
}
public String setup(Customer customer, String nav){
this.customer = customer;
return nav;
}
public void update(){
customerService.update(customer);
}
}