Can I map formula fields in Salesforce?

Mailchimp Field Mappings use Apex Triggers in Salesforce. Triggers, like record-triggered flows, require an edit on a record to run automation. They run on every record edit to update data based on the Mapping Rules.

Formula fields display information when you look at them, but they do not cause a record to be updated. Therefore, Field Mappings won't trigger if the only change on a contact or lead is from a formula.

You can map formula fields with the Data Wizard, however. Since it runs on a schedule or on-demand, it will  pass the value in the formula when it runs.