I took my QX30 to a parking lot, this morning, and did some testing. No "knocking" sound when reversing at full lock.
I'd say this is not normal, and not a quirk of the QX30 chassis/suspension.
Have they checked the mounting bolts on the steering rack to ensure they are properly torqued? Have they checked the tie rod ends for proper torque and excess play?
Are you running OEM tires?
2018 Infiniti QX30 Premium AWD - Majestic White
2019 Volvo XC60 T6 AWD Inscription - Onyx Black Metallic
2018 Honda Gold Wing Tour - Pearl White