Marking a field as mandatory fails to trigger validation for form fields

I marked a field in the .cnd as mandatory (not a nested node)

[myproject:externallink] > hippostd:publishable, hippostd:publishableSummary, hippo:document orderable

• myproject:name (string) mandatory
• myproject:description (string)
• myproject:url (string) mandatory
• myproject:title (string)

and in the document node type definition also marked as true for mandatory

<sv:node sv:name="hippo:field">
<sv:property sv:name="jcr:primaryType" sv:type="Name">
<sv:value>hippo:field</sv:value>
</sv:property>
<sv:property sv:name="hippo:mandatory" sv:type="Boolean">
<sv:value>true</sv:value>
</sv:property>
<sv:property sv:name="hippo:name" sv:type="String">
<sv:value>url</sv:value>
</sv:property>
<sv:property sv:name="hippo:path" sv:type="String">
<sv:value>myproject:url</sv:value>
</sv:property>
<sv:property sv:name="hippo:type" sv:type="String">
<sv:value>String</sv:value>
</sv:property>
</sv:node>

although the fields display as if mandatory (* marked), pressing 'save' in the cms editor showns no validation message when the field is empty.