You are adding multiple lines on an invoice for an exempt customer but only the first line is reflecting that the customer is exempt. Although tax is not calculating, the field 'Taxable Amount' is showing your total order amount as Taxable. The total should be in the field 'Nontaxable Amount'.
- Check the Ship To location to ensure that the address associated to the code is valid
- An invalid address will cause this behavior, although no tax is calculating