PrimeFaces - Validation error in 2nd dropdown (dependent selectOneMenus) - RequestScoped - ajax

Question

The 2nd dropdown is being populated by AJAX, but there's validation error in it. Even if the list is populated correctly and a selection is made.
<h:form id="form">
<p:growl id="growl" showDetail="true" sticky="true" globalOnly="true" />
<h:panelGrid columns="3">
<p:outputLabel for="nome" value="Nome:" />
<p:inputText id="nome" value="#{estadoMB.estado.nome}" required="true" validatorMessage="Nome incorreto!" />
<p:message for="nome" />
<p:outputLabel for="sigla" value="Sigla:" />
<p:inputText id="sigla" value="#{estadoMB.estado.sigla}" maxlength="3" size="3" required="true" validatorMessage="Sigla incorreta!" />
<p:message for="sigla" />
<p:outputLabel for="continentes" value="Continente:" />
<p:selectOneMenu id="continentes" value="#{estadoMB.continenteId}" validatorMessage="Selecione um continente!"> <!-- required="# {not empty param[submit.clientId]}" binding="# {menuContinentes}" -->
<f:selectItem itemValue="" itemLabel="-- Selecione --" noSelectionOption="true" />
<f:selectItems value="#{continenteMB.continentes}" var="continente" itemLabel="#{continente.nome}" itemValue="#{continente.id}" />
<p:ajax event="change" listener="#{estadoMB.onPaisesChange}" update="paises" />
</p:selectOneMenu>
<p:message for="continentes" />
<p:outputLabel for="paises" value="País:" />
<p:selectOneMenu id="paises" value="#{estadoMB.pais.id}" validatorMessage="Selecione um país!"> <!-- required="# {not empty param[menuContinentes.clientId]or not empty param[submit.clientId]}" -->
<f:selectItem itemValue="" itemLabel="-- Selecione --" noSelectionOption="true" />
<f:selectItems value="#{estadoMB.paisesCarregados}" var="pais" itemLabel="#{pais.nome}" itemValue="#{pais.id}" />
</p:selectOneMenu>
<p:message for="paises" />
<p:commandButton action="#{estadoMB.save}" value="Salvar" update="#form" /> <!-- binding="# {submit}" -->
</h:panelGrid>
</h:form>
Method onPaisesChange:
public void onPaisesChange() {
if(pais != null) {
Map<String, Object> map = new HashMap<>();
map.put("id", getContinenteId());
paisesCarregados = paisBean.findWithNamedQuery("Pais.findByContinenteId", map);
}
else {
paisesCarregados = new ArrayList<>();
}
}
Already tried EL binding; required="true" in both and I got the same result.
I think it's not possible to do what I want using request scoped beans...