Unsaved form submission for Mailchimp contact that already exists

Site URL: https://www.irvineclt.org/interest-list

My site is set up with a form block that saves to Mailchimp. It works great. However, I received an email today that I had an unsaved form block submission. After looking into it a bit, I found that the contact that has submitted the form was already in my Mailchimp audience and that they had unsubscribed from my list about a month ago. It appears as if they are trying to resubscribe to the list now. Would them being an existing and unsubscribed audience member cause the form block not to save properly to Mailchimp? If so, is there any way to override this behavior or do I need to manually go in to Mailchimp to update their subscriber status?

Thanks!