Some fields are excluded when Heroku Connect syncs data from Postgres to Salesforce
This can happen when inserting a record. A field will not be written to Salesforce if the Heroku Connect integration user does not have permission to write to the field in Salesforce. To change this, adjust the field-level security settings of the object being mapped in Salesforce.
Heroku Connect doesn't refresh its definition of Field or Object permissions in Salesforce automatically. To resolve this, visit the edit page for the mapping in question. Heroku Connect will get a refreshed definition of the newly changed Field(s) from Salesforce.