I think you need to take the car to someone who specialises in setting up suspension geometry/steering. Assuming the toe, camber & caster are okay on your car, one of other the settings on the sheet I sent you is 'steer ahead'.
Now, I remember having a chat to an 'old-timer' mechanic in the know about this years ago. I understand that steering racks have to be 'centralised' otherwise they can cause the car to pull to one side or the other. I'm afraid I don't know what the process is exactly, but an educated guess would be that the rack is set in the centre of its travel, and the steering rod ends then adjusted to set the wheels in the ahead position (with the appropriate toe-in/out) and then finally the steering wheel set in the centre position.
The chap I spoke to said that most garages/dealers hadn't a clue about this, which explained why this fairly common problem seemed to go unresolved in many cases.
I can imagine this not being done properly at the Alfa factory, & I wouldn't expect a main dealer necessarily to be able to sort it out, so it really is worth checking it out with a specialist.
The only other thing I can think of is a soft damper on one side (I know the car is new, but it could be a faulty component), or a slightly grabbing caliper on one side.
Keep the forum updated on your progress, as I think it is a farily common fault.