When entering a sales order, the tax code field at the line level shows only the tax codes of the transaction subsidiary country, even if there is a different country in the customer record.
The tax code list will display the tax codes of the country specified in the customer’s address by default if the address includes a country specification. If there is no address country, then it will consider the subsidiary country to show the tax codes.
But if the customer address country is not entered as the Nexus in the transaction subsidiary record, then that country will not consider for the tax code field.
So add the customer address country in the subsidiary nexus and when creating the sales order, the tax code list will show the corresponding tax codes(not the tax codes of subsidiary country)