Not able to update the Invoice header amount for a Paid Invoice.

Summary:

We are not able to update the Invoice header amount for a Paid Invoice. In some cases, the user needs to update the freight information to the invoice after it is paid and then pay the freight amount.

We have enabled "Allow adjustments to paid Invoice" check box in Invoice options. We were able to add a Freight Line at the invoice line level, but we are not able to edit the header amount as it in Frozen. Added screenshot below.

Is this the intended functionality? If yes, why the system is allowing us to update the line information if we cannot update the header information.? Please share your inputs