The recent update treats the county as a necessary component of a complete UK address, s-jackson.
When you create an invoice, the system will autofill the code based on the selected county in the customer-facing address. This information is automatically converted into a two-letter code and displayed on the invoice.
For now, there's no option to turn it off or remove the code when creating sales transactions in QuickBooks Online.
Let me know if you need anything else.