HSDL National Identifier is Requiring Issue Date

Background: I am attempting to create an HSDL template to load our new student workers in HCM with their assignment and person details. I've almost completed this template, but I'm facing issues with the National Identifier SSN load.

Issue: As far I can tell from the Worker Business Object documentation, Issue Date should not be required, however, when I add any of the required Person National Identifier fields to my template, Issue Date is also brought in as required. Why is this?

You can see in the above screenshot that Issue Date is not required. If I attempt to upload the HSDL with Issue Date as blank, I get the following error