we have a Marketing Cloud email with a custom unsubscribe link. When clicking on this link, contacts are redirected to a cloud page where they can manage their preferences and, if willing, perform a complete unsubscribe. Since we are using this custom link, the Marketing Cloud standard unsubscribe link is not present in the email. However, some contacts seem to have performed the standard unsubscribe. They didn't report the email as spam, but when looking at the tracking of the email we can see that they unsubscribed from the Subscription Center. When using the custom link, contacts are not listed among the unsubscribes in the Tracking Sends report.
We cannot understand why we see these contacts as having performed the standard unsubscribe. Could it be that blocking the sender in their inbox (without reporting the email as spam) is tracked in Marketing Cloud as an unsubscribe from the Subscription Center?