I ran into a situation where inserting addresses into accounts vs custom objects does not have the same behavior for state codes. For accounts, US state codes are not accepted and I can only enter the state if it is the full state name, while the custom address field on custom objects can accept valid state codes or the full name.
Is this intended behaviour? If so, why are they treated differently?
In case of State and Country Picklist feature is Enabled,
Billing State Field will use Integration value, Admin can set this value for any given state/country in Configure States, Countries, and Territories page.
You can refer help doc for more details.
If you like to use codes for Billing Address field you can use "BillingStateCode" field instead of "BillingState".
Custom Address field will always work with codes.
