If you see a notch about 2 mm deep on the Brembo rotor you bought, there's nothing to worry about as this is not a defect.
This notch is simply where a small quantity of material has been machined from the rotor during production to ensure that the rotor itself is perfectly balanced - as any imbalance can produce undesirable vibration.
All rotors are checked and, if necessary, balanced automatically during production.
Rotors are checked on an electronic test bench which spins the rotor and measures any imbalance. If any imbalance is detected, the excess material is ground away to balance the rotor perfectly.
Not all rotors need correction, and the majority are already perfectly balanced, making it unnecessary to machine material from the outer edge. This means that some rotors may have a notch while others do not.