I’ve had this issue before. I’m guessing a lot about your setup so correct me if I’m wrong.
I’m guessing you’re using a usb to ethernet adapter to pxeboot? FOG is reading the MAC address of the usb adapter instead of the internal NIC.
I’m not sure if there’s a better solution but - we added the device onto fog as normal. Imaged it then deregistered the device from FOG for the next one. Obviously - not really a solution if you’re relying on fog for asset management.