When working with Brevo contacts, the fields Attributes_FirstName, Attributes_LastName, and Attributes_Phone are provided as standalone fields for convenience, as they are universal to all users. However, for adding or updating custom fields, you must use the primary Attributes field exclusively.
If Attributes_FirstName, Attributes_LastName, or Attributes_Phone are mapped, they will take precedence and overwrite any corresponding values defined within the general Attributes field.
For custom field imports, map only the Attributes field and leave Attributes_FirstName and Attributes_LastName unmapped.
The Attributes value in your source must follow this JSON format:
{"LASTNAME": "LName", "FIRSTNAME": "FName", "TITLE": "Title"}