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Make sure that MC4WP “knows” about the field, else it will ignore it.
Go to MC4WP > Mailchimp, and click “Renew Mailchimp lists”.
That will pull in the latest changes. You can confirm by clicking on the list name to fold it open and you should see the field there. You can also triple check the field name in that same screen.
Hope that helps. If you have any questions, please let me know!