You're creating or updating a mapping in Heroku Connect. When you map a field you see a message saying "Calculated fields are not syncable: ".
Calculated fields (also known as, Formula fields and Roll-up fields) do not always update the Salesforce system fields that Connect uses to synchronize these fields (
LastModifiedAt). This means that while an initial load may successfully "synchronize" the data, the data may not stay synchronized after an initial load.