I have found a possible flaw on both the XTS and CT6 that IMO, should have been caught by Quality Control that probably no owners have notice.
On both the drivers and passenger side at the rear wheel well at the point that the bumper meets the rear wheel it comes to a point. Directly above that point there is an imperfection on the body of the car that gives the appearance of a dent. It is hard to see because it almost looks like it is just the way the light reflects on that area but it is a bend in the metal. The best way to see the imperfection is to look at that area at an angle. You can also feel the area and you will be a able to feel the bend. It is more noticeable on the passenger side than the drivers side. Once you see it you will know that it is there. It is something that is better off left alone than try to have corrected but I am surprised that it was not caught by quality control.
If you have notice it and thought perhaps someone hit your car with their door, probably not, what you may be seeing is this imperfection that is probably due to something that probably was not quite right when that rear panel was formed.
I checked other 2019 XTS and CT6 on the dealers lot and found the same. I also saw the same on my brother in laws new 2019 XTS.