Because a Person Account has no separate child Contact, the automatic address flow down does not apply to it. Automatic flow down copies an Account’s Billing Address down to its related Contacts, and a Person Account has no related Contact to copy to.To keep a Person Account’s addresses in sync, use the manual Sync Address Details button. It copies the Person Account’s Billing Address into its Mailing Address on demand.For how automatic flow down works on regular Accounts and Contacts, see Account Address Flow Down.