With regards to using the product flat line item, did you intend to get the display name (from A2 numbered list) into the Product flat line item and then use that in the FINDITEM function? It populated the data as needed in the P2 Product with no issues. I guess it doesn't matter if we use ID line item or product flat line item. It can be achieved either way by using the display name from the A2 list. I've attached the screenshot for your reference. Please let me know if you had another optimized way of doing it.
@ssicefox In finditem(list, text formated lineitem) function you are asking anaplan to find the list item in the list based on the text formatted line item which can be (display name or code) property of the list