Field mappings not updated from Bulk API

We have a custom object (ID 31) which has been mapped to Contact object via email address field. We have also defined Field Mappings under the custom object to update certain Contact fields whenever a Custom Object record is created.

The above functionality works as expected when we create a new record under Custom Object. The mapped fields of corresponding Contact are also updated.

However, when we create a new record using the Bulk API, the corresponding Contact fields are never updated. Although the new Custom Record shows as "Mapped=yes" and in the logs I can see that it has been linked to the contact record.