I also have this problem with my X on 2019.5.16.
I was also wondering why would they program my car to slow down and match speed of a car in an adjusted lane even if that car is not getting into my lane.
But I think I've found what they are trying to achieve.
One day I was driving on a freeway in a most right lane (I rarely do so) and there was a car in a merging lane entering the freeway. I was on Navigate on Autopilot and was thinking that I have to disable the autopilot to let the merging car in as it was kind of ahead of me a little bit and moving with almost the same speed and autopilot is not going to allow this car in as it's not reacting to cars in other lanes. To my surprise, my car started slowing down clearly reacting to the car in the merging lane. It slowed down to just allow that other car to be ahead of me and giving it enough gap to merge, then matched my speed with the other car speed. After that, the other car was able to merge into my lane just perfectly fine. All this was happening just perfectly, with no sudden brake or sudden acceleration. It was very nicely done and I probably would do exactly the same myself!