HERE IS THE ANSWER:
Simply, you cannot target a camera exactly vertically downwards.
You need it at an angle so the camera knows which way is 'up' and which is 'down' and which direction to face - even though it appears to be vertically downward.
So try this:
_camera camsetpos [_x,_y,10]
_camera camsettarget [_x,(_y+0.01),0]
Effectively you will get the same thing.