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The 91 status identifies list subscriptions that are removed from contacts — not unsubscribed, but actually removed from the contact record — this is possible and happens when the contact_edit v1 API endpoint is called, because that endpoint enables the "overwrite" parameter by default.
In "overwrite" mode, the contact_edit endpoint replaces all existing list subscriptions for the contact in question with whatever set of lists is passed in the "p" parameter - meaning it will unsubscribe the user from any list the API consumer did not send along in the API call.
If you're using the contact_edit endpoint make sure to set the "overwrite" parameter "0" in the API request to avoid overwriting the list subscriptions in the contact record.
If you are not using the API yourself, you should contact the integration that you are using and request them to fix their usage of this ActiveCampaign API endpoint.