It has not bothered me so much. I have the c1 button programmed to center focus, and the way I hold the controller makes me often "accidentally" hit it which constantly ensures me that the scene is in focus. Im actually considering changing this button, to stop the random auto focus change in the middle of a shot. The focusing on the desired point with the touch screen has allready entered my muscle memory every time I start a recording, so it should not be a problem.
Another thing to consider, is to try using manual focus, since I want to focus on a foreground object, and then raise the aircraft above it, and focus on the horizon. I haven't tried this yet, but I've tried to change the focus to the horizon using the auto function, which looks really ugly in the video.