If you add a contact via Primary Contact, then the created contact record does not have a default contact class assigned. However, if you create a new contact from the Contacts screen, then it defaults the contact class
Date: 29/04/2024
Status: Fixed in Acumatica Build 2024R2Beta, will be merged into upcoming release 2024.2
Reference: CE00047113
Issue
Contact Class not defaulting when created through Primary Contact on Customers screen (AR303000)
Workaround
Manually add the contact class