This is an old bug, been there forever, you will need a custom line field to source the value from the Item record and use that custom field.
• This ${item.item} is available at the transaction level in the item sublist.
• This ${item.description} is available at the transaction level in the item sublist.
• This ${item.item.purchasedescription} is not available at the transaction level and is trying to get the value from the Item record and that does not work for Advanced PDF templates, it would repeat the first line value across the board.