Yeah I'm going to have to call "Shenanigans" on this one. Exception would be after washing the vehicle. Our 2016's brakes will squeal under pressure for a few rotations if I have washed it and let the water dry on the rotors. It forms a bit of surface rust which causes the noise but quickly goes away. Did it do this when you test drove it? You may have something stuck to one of the pads but the dealer would be able to resolve that for you. I live in a hood with a lot of Porsches. None of them make noise under braking. Take it to a different dealer if possible.