In AP, I enter an invoice matched to a PO. on the invoice line, the tax rate has defaulted from the

In AP, I enter an invoice matched to a PO.  on the invoice line, the tax rate has defaulted from the site.  But I change it to the correct rate. I then click on calculate tax.  The item line has the correct new tax rate name against it, but a tax line has generated based on the old tax rate.  The tax details form for the line still references old rates.  To fix the invoice I have to go in to the tax details form and update the tax status.  Why doesn't the invoice calculate tax based on the rate