Sales Order User Fields in Invoice headers not being persistent

Hi, 

I have and issue with the UDF in Sales Orders not being saved after an Invoice is processed.

i enter the user defined fields in the sales order and press place order in the sales. The UDF are saved correctly.

Then I open the sales order and process and Invoice.

On processing the invoice the UDF show up correctly on the Invoice but when i go back to the Sales Order that originated it, it is not there and the user defined fields are all empty.

i need the UDF to be persistant over processing the invoice as many invoices can be generated from one invioce

We are using 9.20.1.003.

The UDF are in the headers of the document.

Regards, 

Dino