Should you be able to update invoice amount AP Forms for invoices that originate from iSupplier?

We have some ISP invoices that have incorrect invoice amount due to a problem we had with our 3rd party tax vendor.  The invoice amount does not match the sum of the invoice lines and so they are on line variance hold.  The fix is to update invoice amount, however the invoice amount field is grayed out in AP Forms for invoices that originate from iSupplier. (this issue occurred for other sources of invoices too, but AP Business area has been able to correct via AP Forms by updating invoice amount).

I have an SR opened on this issue, but i'm not getting response to this question: