Does this happen (member being unsubbed) after just one such email deleted from spam?
It can. ?
Does it happen more to certain types of email addresses?
Yes. ?
Not every email service implements a "feedback loop" (that's what this "feature" is called) to inform list managers of spam markings by list members. And those that do appear to have differing amounts of strictness.
Ideally the mechanism would require that the email user explicitly mark the message as Spam, and then be offered the choice of being automatically unsubscribed. But it has been observed that some email services will trigger the mechanism even when a message was automatically delivered to the Spam folder, and then automatically deleted from there - with no user input nor notice to the user.