You might want to check the answer on this one: acumatica - Updating default contact on customer adds new record - Stack Overflow
You will want to target the BAccount record as the base graph, and then update your Contacts relative that record. The Additional Account Info is part of the DefContact record. There are some “default” contacts for Customer records that live behind the scenes and implement an inheritance scheme. MainContact is the primary contact object, and it contains the address info in the “General” tab of the Contact screen. Then there are the Billing and Shipping contacts, which will inherit from Main Contact, unless the “Override” option is chosen on the Billing or Shipping screens. The info you want to update is located in the “DefContact” record, which is another behind the scenes contact record for the “Additional Information” section of the Customer record.
So I think targeting FieldUpdated is fine. But these records reside in different contact objects, and unless Additional Info has been filled out yet, the DefContact might not even exist yet. So you will have to manage populating DefContact and the Primary contact when the other is created or updated.