When a contact unsubscribes from emails in ConvertKit, ConvertKit prevents the subscriber from being accessed via the API.
This means that any tags associated with the subscriber will no longer be available in WordPress.
Unfortunately the only way around this is to manually delete the subscriber and re-add them, then manually apply their access tags.
Under the Advanced tab in the WP Fusion settings you can choose to be notified via email when a subscriber unsubscribes.
When this setting is enabled you’ll receive an email at the specified address containing the email address of the unsubscribed person. You can then log into ConvertKit, delete them, and add them again.