I went to the Update Line Items form.
Focus is on the Product field.
Simulated the barcode reader by pressing "6" on the keyboard, then Enter.
Enter triggers the Save button, which is expected as it has "Save form on enter" set.
When inspecting the record I see that the correct product (Draw, ID 6) was captured.
The Name and RRP fields are empty. (Because the "save" happened before the "Additional fields").
I turned off "Save form on Enter".
Now if I press say 7, Enter then the Message Box product appears, and the other fields are filled in.
I guess if there's something wrong in all of that, you need to walk me through what you are wanting to do differently, and what you are wanting to see.
Cheers
Bruce