Reason and logic? The fact that the object was in the air is irrelevant. It wasn't moving so "a relatively slow moving person or car" is still moving infinitely faster than the aircraft I was tracking. Even if it had been moving I have the ability to control the speed at which the aircraft moves. It's possible for a VTOL aircraft, which quadcopters are, to move slower than cars and people typically do. Nothing in the documentation indicates that tracking an airborne object should be different than a land or waterborne object.
I agree - it has nothing to do with the speed of the object, at least in this case. As mentioned previously, the problem is almost certainly that the tracking algorithm expects the motion of the tracked object against the background to represent the actual motion of the object. That's essentially true for an object on the ground. However, for an object in the air, the apparent motion against the background is a combination of the actual motion of the object and the parallax change as the viewpoint changes. It almost certainly has no way to separate those, and that's going to lead to strange effects when it tries to follow.