OrderNumber is optional, but get error OD0042: Duplicate OrderNumber

Using P21-validator 2.1.0 on a define-xml v.2 file.

Although my define.xml file does not have any "OrderNumber" attributes at all, I get a warning OD0042 "Duplicate Order number" for each of the variables. I consider this as false positives as the Define-XML 2.0 specification states:

3.4.1. Use of the OrderNumber Attribute
In several locations, the Define-XML specification uses an optional OrderNumber attribute to define the relative ordering of elements within a container. In the context of Define-XML, if the OrderNumber attribute is specified on an element, all elements of the same type (element name) within the same parent element must also specify an OrderNumber attribute. It is an error to mix elements with OrderNumber and without OrderNumber within a given parent element. If no OrderNumber attribute is specified in cases where order is important, the ordering is derived from the XML document order.