Lock In Letter checked but still allowing Dependent Amt to be updated

Our Payroll Manager had a question about functionality concerning Lock In Letters and the Employee Tax Data page for the 2020 Or Later Federal Withholding Elements

Under Payroll for North America --> Employee Pay Data USA --> Tax Information --> Update Employee Tax Data when she marks LETTER RECEIVED under the Lock-In Letter Details then adds a Dependent Amount, it is not giving an error...If you try to change the Tax Status, it will kick back with an error. She explained to me that the Letter Received option should also lock out the amount as it will change an employee's taxes which isn't allowed.