From Zendesk:

Attempts to submit requests with a staff member role (admin, agent, etc.) identity will fail by design. Apologies for any confusion around this behavior.

The end result is that HEs are unable to test the feedback form. When they try, they see this:

According to Zendesk, a potential workaround for this is to programmatically append "-testing" when the feedback form is submitted from an @automattic.com or @a8c.com email address.

Update: This change should be made for all Zendesk ticket creation, not just the support form.

Related iOS issue