Why is an auto-created package preventing a project from being uploaded?

I'm trying to publish a FileBasedProject to GroupShare (2020) using this example: https://developers.rws.com/studio-api-docs/apiconcepts/projectautomation/connecting_a_project_to_a_project_server.html#publishing-to-a-project-server

However, this fails. The exception says:

System.Xml.Schema.XmlSchemaValidationException: The 'ArchivedAt' attribute is not allowed.

Sure enough, if I open the project file (*.sdlproj) I can see...

I also find that a package has been created.

If I delete that attribute and the package and then try again, both the attribute and the package are recreated.

Why is this ArchivedAt attribute added? Does it indicate the date/time at which the package was created?

What am I doing wrong when I believe I've followed the example?