Summary:
If a CRM contact unsubscribes to your MailChimp campaign via the unsubscribe link the pers_optout field will not be updated to reflect that they have opted out of e-marketing campaigns.
The information concerning their unsubscribing from MailChimp will however still be correctly stored in the MailChimpUnsubscribed table.

Symptoms:
The pers_optout field is not updated when a person opts out of a MailChimp campaign.

Status:
This is a known bug and has been escalated to development.

More information:
As a workaround you can either manually opt each unsubscribed user out of e-marketing campaigns from CRM by going to the Person -> Summary -> Change and check the opt out checkbox or run some SQL like the following directly on the database after users have unsubscribed:

update Person set pers_optout='Y' where pers_personid in (select McUn_PersonId from MailChimpUnsubscribed)