Changing a result from p:autoComplete's completeMethod before using it - jsf

Question

I'm implementing a dialog for registering an address. In the street name field, I'm using PrimeFaces' <p:autoComplete> tag to help the user find the name of their street. The completeMethod works as expected, but to avoid confusion between similar street names I would like the drop-down list to also include the municipality the given street is in, for instance on the format "<street name>, <municipality name>".
I don't want the municipality name to be included in the actual field, so I've concluded that I need a method that performs some sort of string manipulation (substring using the position of the first comma, for instance), but I can't figure out where such a method would be called from. I've had a look through the PrimeFaces documentation, but I haven't been able to find anything that would allow me to do this. Is this at all possible in <p:autoComplete>? Alternatively, is there another autocomplete implementation which supports this, or would I have to implement my own javascript component?
EDIT: This is what the xhtml code I'm using looks like:
<div class="form-group row required">
<h:outputLabel value="#{msgs['#common.mailingAddress']}" for="address" styleClass="col-xs-12"/>
<p:autoComplete
id="address"
name="address"
size="50"
maxlength="50"
styleClass="col-xs-12 street-name"
label="#{msgs['#common.search']}"
disabled="#{not configurationController.cardCtrl.editable}"
value="#{configurationController.cardCtrl.selected.address}"
required="true"
completeMethod="#{configurationController.cardCtrl.autoCompleteTest}">
<f:validator binding="#{onlyLettersOrDigitsValidator}"/>
</p:autoComplete>
<h:message id="addressMessage" for="address" styleClass="inline-error inline-error-small"/>
</div>
The autoCompleteTest method in the controller is as follows:
public List autoCompleteTest(String input) {
AddressSearch addressSearch = AddressSearch.builder()
.streetName(input)
.municipality(municipality.getName())
.maxResultsPerPage(10)
.build();
return addressesToStreetNames(mapService.addressSearch(addressSearch).getAddresses());
}
With a helper method addressesToStreetNames which takes a list of Address objects and returns an ArrayList<String> containing those addresses' street names.
EDIT2: Based on suggestions in the comments, I tried setting itemValue and itemLabel to different values, to see if that had any effect. The new xhtml looks like the above, with the addition of the following three lines:
var="address"
itemValue="#{address.streetName}"
itemLabel="#{configurationController.cardCtrl.formatAddress(address.streetName, address.postTown)}"
The autoCompleteTest method now also returns the Address object directly rather than a String representation of the street name, so that these fields are available. The formatAddress method is simply return streetName + ', ' + postTown;
This causes the dropdown list to look how I want it to look, but when I click an item it still inserts the whole string with both street name and post town/municipality into the text field (and in fact, before I've written anything, the text field already contains ", ").