Harbor is my go to supplier for sheet goods as well, and I have seen similar issues from time to time.
It happens everytime they change their routes and or replace drivers.
Not sure if NJ is the same route, likely not but our products make at least one truck change from maryland to harrisburg to our delivery truck.
In the past when they dropped off well before we were in, at 0 dark thirty the problem was bad, a single driver delivering a case of gator or pvc and the cases were often corner damaged or split open. Now it is an occasional problem, but within the tolerance for the money and buying single sheets of weird things often. Cant imagine another supplier pulling off what they do as well as they do it...
They have never challenged me when I ask for replacement or adjustment.