Why PO interface won't update some header columns for updated PO with additional lines?

Hello there,

I have a question on PO Interface process and here is the detail information:

1) We tried to use two columns, note_to_vendor and note_to_receiver in PO_HEADERS_ALL in our PO interface process and everything works

2) However, we found the values for the two columns only got updated for any new POs but not for updated POs with additional lines. For instance, original PO has a line and when we add a line to the PO, the line is added but the two columns in the header won't get updated with the values that are already in po_headers_interface. The interface process just ignores the values