In the custom field line if the first item has an empty field the whole line would not show up on preview or print page.
Photo show an example of an invoice i made today and i could not show serial number on printed page so i moved an item with serial number to the top to fix it.
Ya it’s weird, today i tried something i removed the item from inventory for the first product and it worked when i re-add the inventory item the issue appeared. I will create a dummy file and uploaded it.