Primefaces Client Side Validation not displaying message in Growl

Question

I am tying to use the primefaces client side validation framework. Most of it is working correctly, but for some reason the validatorMessage doesn't display in my growl. When I just have the server side validation it appears fine. I have added a standard message and this also appears fine, it just won't appear in the growl.

My growl is defined as:

<p:growl id="errorMsg" showDetail="true" sticky="true" autoUpdate="true" widgetVar="growlErrorMsg" />

A snippet of my xhtml is:-

            <div class="field">
                <p:outputLabel for="Amt1"
                    rendered="#{MyWebPage.amt1.isRendered()}"
                    value="Amt: "
                    binding="#{MyWebPage.amt1Prompt}" >
                <span class="required">*</span>
                </p:outputLabel>
                <p:inputText id="Amt1" maxlength="10"
                    validatorMessage="Input must match this pattern: -ZZZZZZZZZ9"
                    value="#{flowScope.amt}"
                    binding="#{MyWebPage.amt1}"
                    disabled="#{MyWebPage.isDisabledAmt1('')}"
                    styleClass = "numeric" >
                    <f:converter converterId="zero" />
                    <f:validateRegex pattern="^-?([0-9]{1,10})$" />
                    <p:clientValidator />
                </p:inputText>
                <p:message for="Amt1" />
            </div>

The code above includes the <p:message> which displays successfully. Without this, the field error is still highlighted but no message appears.

I am using Primefaces 6.0, JSF 2.1.13 (Mojarra), and running on Tomcat 8.5.4.

Any guidance would be much appreciated.