InboxSys calls the headers of all links in all seeds, in order to test if links work. InboxSys is not calling the body of that link! If this little check unsubscribes immediately, that indicates a few severe flaws in the unsubscribe mechanism.
First of all, no-one uses a single-click unsubscribe in the bottom of an E-Mail. It's legally allowed to use a dual-click unsubscribe, so that is the common standard. When you click on an unsusbscribe link, the landing page should show a confirmation dialog. Otherwise, you get bot-unsubscribes! That is basically what we have here: a bot-unsubscribe.
Secondly, we are calling the header of the page, but not the body. Even if there is no dialog, calling the header should only send back header information, but not execute actions that are typically taken after the full page has loaded.
An immediate unsubscribe upon only calling the header of a page looks symptomatic to a List-Unsubscribe link. LU have opposite requirements and best practices from standard unsubscribe links. InboxSys never calls a List-Unsubscribe link, for this exact, obvious reason.