I have a top of the range Canon HF G10 which even has dedicated fast autofocus sensor
there's the problem. top of the range performs worst than low fo the range. i suspect either lemon camcorder or user/setting error. please check in the menu if anything related to autofocus you've accidentally set such as focus area, priority, mode etc. i noticed your camcorder tend to stick to the focus of the previous/farther object such as background at the side/top of the picture/frame or reflection of lamp from lcd screen. iirc in quadcopter video.
edit: and try to play around such as tapping the manual focus ring (slightly rotate) while shooting, if it helps redirecting focus point.