Setting values of interdependant fields from backing bean

Question

I am using JSF 2.2 (Mojarra), PrimeFaces 4, Tomcat 7 I have two interdependent Autocomplete fields in a ui:repeat. When I select something in the first autocomplete, I need to set the value of the second and vice versa.

<ui:repeat var="anItem" value="#{aBean.itemList}" varStatus="status">
    <p:inputText id="someId1" value="anItem.operation" required="true"/>

    <p:autoComplete id="nature" dropdown="true" forceSelection="true" value="#{anItem.nature}" completeMethod="#{myController.completeNatureList}">
        <p:ajax event="itemSelect" listener="#{myController.selectNature(status.index)}" process="@this" update="code"/>
    </p:autoComplete>
    <p:autoComplete id="code" dropdown="true" forceSelection="true" value="#{anItem.code}" completeMethod="#{myController.completeCodeList}">
        <p:ajax event="itemSelect" listener="#{myController.selectCode(status.index)}" process="@this" update="nature"/>
    </p:autoComplete>
</ui:repeat>

My beans

@ManagedBean(name = "aBean")
@ViewScoped
public class Bean implements Serializable {

    List<Item> itemList;

    // getter and setter
}

public class Item implements Serializable {

    private String operation;
    private String nature;
    private String code;

    // getter and setter
}

And my controller looks like this :

@ManagedBean(name = "myController")
@ViewScoped
public class Controller implements Serializable {


    @ManagedProperty(value = "#{aBean}")
    private Bean aBean;

    @Override
    public void selectNature(final int pIndex) {
        LOGGER.debug("Nature selected");
        // business logic
        this.aBean.getItemList().get(pIndex).setCode("code from business logic");
    }

    @Override
    public void selectCode(final int pIndex) {
        LOGGER.debug("Code selected");
        // business logic
        this.aBean.getItemList().get(pIndex).setNature("nature from business logic");
    }
}

The first time I select something in the first Autocomplete, it works fine. But if I then select something in the second Autocomplete, my first Autocomplete is not updated. And if I reselect something in the first Autocomplete, this does not work anymore. The logs tell me my controller is always called.

I tryed to update the whole form and it works every time, but I cannot afford that of course, because of all the required fields in my form.

<p:ajax event="itemSelect" listener="#{myController.selectNature(status.index)}" process="@this" update="@form"/>

So my model is correctly updated, the problems seems to be the update of an already processed field in the render phase.

I do not wish a workaround because I have plenty of similar cases. I would like to understand why JSF behave like this and what I did wrong, any idea why ?

EDIT

Looking at this post How to address a component inside a looping naming container, I tried with a c:foreach instead of a ui:repeat, and it works !

So now I would just like to understand why my old solution with ui:repeat only worked once.