The attribute is defined like this in the schema:

  <xs:complexType name="UANode">
  ...
    <xs:attribute name="AccessRestrictions" type="AccessRestriction" default="0" />
  ...
  </xs:complexType>

This attribute exists also in the model tag as a namespace default in the same way as for RolePermission.
When loading the XML file a missing AccessRestrictions attribute means to be replaced with 0.
So there is no way to determine if a namespace default should be used.

By removing this default value the problem can be resolved. A missing attribute then means that the namespace default value should be used.