I can provide information regarding my Hyundai Tucson, which is equipped with the HDA capability, and my eldest son own a similarly equipped KIA EV6. HDA operates quite well on both vehicles.
I recommend that you schedule an appointment with your selling dealership and elucidate the issues encountered with the HDA engaged, as it should not function as you have described.
If you purchased the car as new, you have the option to reject it; the procedure is available online or through Citizens Advice.