Samsung say may warranty is void from rooting, I did not root!

I've had my note 4 for five months and it began rebooting, so I sent it to Samsung for repair. They stated "device’s KNOX warranty has been invalidated by rooting" The phone was brand new, I didn't root the device, as I'm well aware that a rooted phone will void the warranty. It's on Lollipop.
Any advice would be greatly appreciated.
I spoke to the engineer who wouldn't go into specifics and passed me back to Samsung customer care.