PrimeFaces Command Button vs. Default Command Butt

When I am using a PrimeFaces command button

<p:commandButton action=#{bean.action} />

I don't see the the validation messages for inputs (both the default h: ones or the PrimeFaces p: ones.) For example with

<f:validateRequired />

When I am using default command button like

<h:commandButton action=#{bean.action} />

I do see the validations. What can be the cause of this difference?

I am using Prime Faces 3.5 with Mojarra 2.1.18

<h:form id="reliefhourheadcopy-form">

        <h:panelGrid columns="1">
            <h:outputText value="Kopiere Entlastungsstunden von" />
            <h:outputText value="Semester: #{reliefHourHeadManagedBean.reliefHourHead.semester}" />
            <h:outputText value="Jahr: #{reliefHourHeadManagedBean.reliefHourHead.year}" />
            <h:outputText value="nach" />               
        </h:panelGrid>

        <h:panelGrid columns="3">

            <h:outputText value="Semester:" />
            <p:selectOneMenu id="semester" value="#{reliefHourHeadManagedBean.semester}">
                <f:selectItems value="#{reliefHourHeadManagedBean.semesterTypes}" />
            </p:selectOneMenu>
            <h:message for="semester" />

            <h:outputText for="yearSpinner" value="Jahr:" />
            <p:spinner id="yearSpinner" value="#{reliefHourHeadManagedBean.year}" maxlength="4" min="2000" max="2030" size="4">
                <f:validateRequired />
                <f:validateLongRange minimum="2000" maximum="2030" />
            </p:spinner>
            <h:message for="yearSpinner" />

        </h:panelGrid>

        <h:panelGrid columns="1" style="margin-top:25px">
            <p:commandButton action="#{reliefHourHeadManagedBean.copyReliefHourHead}" value="Kopieren" icon="ui-icon-copy" >
                <f:param name="reliefhourhead_id" value="#{reliefHourHeadManagedBean.reliefHourHeadId}" />
            </p:commandButton>
        </h:panelGrid>

    </h:form>

The primefaces commandButton will submit the form by default. However, in order to get the form updated you need to add the update attribute:

<p:commandButton action="#{reliefHourHeadManagedBean.copyReliefHourHead}" value="Kopieren" icon="ui-icon-copy" update="@form">
                <f:param name="reliefhourhead_id" value="#{reliefHourHeadManagedBean.reliefHourHeadId}" />
            </p:commandButton>