I had this issue on my 2016 CT6. The plastic (or whatever) splash shields that protrude downwards and just forward of the rear wheels split and became detached. I simply used epoxy to glue them back in place and that held fine for the rest of the time I had the car. I also put a dab of epoxy on the clip joints.I have checked my 2019 (CT6 #3) and I haven't noticed the imperfection as described. Now I wish I hadn't looked, because I discovered a loose gap between the entire plastic lower trim panel and the upper rear fender. These vehicles are hung together with plastic clips. It's a large body panel wraps from the wheel wells all the way around the rear bumper. I was hoping that I would get a CT6 that stays intact within its first 3,500 miles, but at least this is a minor flaw that can be repaired at its next service visit, I hope.
Unfortunately the design is the same on my 2019 CT6. So far (4000K) all is OK but I have the epoxy ready if and when.
Regarding the rear fender "imperfection" issue....my CT6s did not show that. However the front left fender on the 2016 had something similar...it was like the fender did not align exactly right and they had to force it into place. Keep in mind that we are talking Cadillacs here which frankly do not have the same body fit and finish as the Germans and the Japanese.
That said, I am immensely enjoying my 2019 Platinum....it has an incredible balance of luxury and sport driving capability that I would have thought would be impossible to achieve. Hats off to the GM engineers in that regard. That is exactly the kind of driving performance that I was looking for. Also...no problems at all to report AND the transmission is as smooth as can be! Really lovin' it! Sometimes I just take this car out to drive it around as if I was a teenager with my first car....it has been a very long time since I did that with any car.