Is it normal for foam core white backing to wrap and twist slightly? Yes!
Prints come in a tube or flat. When coming flat, the print is backed by resilient foam core that protects the mat and print from moisture and potential damage. It is normal that the white foam core may come slightly warped or twisted, as its purpose is to absorb moisture away from the print as heat and cold contract and twist the backboard slightly. This will all go away once you have properly assembled a frame and use this backboard as it is in tact and in place around the perimeter of the frame. No foam core is perfectly flat until it is in the perfect temperature environment.