Odd field behaviour in page extension

Hi folks,
I have an odd error with a simple page extension for the Sales Order page.
Essentially, the extension shows a couple of fields, changes the caption of one of them. I published that from vscode and edited in Designer.

Some Sales Order records show these fields fine, others do not. There is no underlying pattern with the Sales Order header data or lines I can see, however these orders were all created by one staff member…

If I look at a Sales Order which is not showing the “External Document No.”, Designer tells me the field has been placed, but it isn’t showing on the page at all. If I drag the field out to the page and download source to vscode again, it is showing a “duplicate” field on the page. Now some records show two copies of the field, but at least the offending Sales Orders are now showing the field…

I’m at a loss - there is nothing in my code which could conflict with this extension, I’ve checked it thoroughly. I feel like there is something here I’m missing or misinterpreting - any help would be greatly appreciated.

Any ideas please gurus?
Page extension excerpt follows:

        modify("External Document No.")
        {
            ApplicationArea = All;
            Importance = Standard;
            Caption = 'Customer Order Ref.';
            Visible = true;
        }
        modify("Quote No.")
        {
            ApplicationArea = All;
            Importance = Standard;
            Visible = true;
        }
        modify("Shortcut Dimension 1 Code")
        {
            ApplicationArea = All;
            Importance = Standard;
            Visible = true;
        }
        moveafter("Quote No."; "External Document No.")
        addafter("Sell-to Customer Name") //this is added by designer
        {
            field("Quote No.59778"; Rec."Quote No.")  //note number after "duplicate"
            {
                ApplicationArea = All;
            }
            field("External Document No.85875"; Rec."External Document No.")  //not sure where the number is from...
            {
                ApplicationArea = All;
            }
        }