icefaces-ace faces-config.xml has typo in the ordering section:
    <ordering>
        <after>ICEfacesCore</after>
    </ordering>
It should be:
    <ordering>
        <after>
                <name>ICEfacesCore</name>
        </after>
    </ordering>

Due to this the order of faces-config.xml files processing depends on the name of jar files (e.g. if the jar has name icefaces-ace-4.0.0-20131219.jar then I get one order, if icefaces-ace.jar then I get another). In the first case the args argument of the onComplete handler is filled correctly, in another case it's always empty. If I fix the ordering section of the icefaces-ace faces-config.xml file, then args is not filled.

This is the stacktrace then args is filled:
CallbackPartialResponseWriter.endDocument() line: 53
DOMPartialViewContext.processPartial(PhaseId) line: 279
UIViewRoot.encodeChildren(FacesContext) line: 981
UIViewRoot(UIComponent).encodeAll(FacesContext) line: 1757

This is then not:
PartialViewContextImpl$DelayedInitPartialResponseWriter(PartialResponseWriter).endDocument() line: 131
DOMPartialViewContext.processPartial(PhaseId) line: 279
AutoUpdateCallbackPartialViewContext(PartialViewContextWrapper).processPartial(PhaseId) line: 183
UIViewRoot.encodeChildren(FacesContext) line: 981
UIViewRoot(UIComponent).encodeAll(FacesContext) line: 1757

Because AutoUpdateCallbackPartialViewContext goes before DOMPartialViewContextFactory in a chain, CallbackPartialResponseWriter isn't called at all.