to end the dispute if that is a crack or not, I would look underneath the playfield and see if the crack is visible there. It would probably start at one of the screws used to secure the target to the playfield

if it does start at one of the screws it was most likely caused by it not being pre drilled. That would most certainly be a production flaw and not due to normal wear and tear or other factors. however things like that can certainly make the crack worse over time.