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 utilize 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 your specific configuration, you should map only the Attributes field; ensure that Attributes_FirstName and Attributes_LastName remain unmapped. Furthermore, to ensure successful processing, the AttributesString in your source must adhere to the following JSON format:
{"LASTNAME": "LName", "FIRSTNAME": "FName", "TITLE": "Title"}