That´s a bug.
If your animation starts from outside of page area, place animated object to it´s starting point and change animation type to From Current Appearance. Object probably still flashes when user enters the page but nobody can see that since it happens outside page area.
If your animation´s starting point is inside page area, you have to create some kind of mask for it. If it´s on white page, simple white rectangle will do fine. Make sure that white rectangle is front of the animated object. When you start your animation, fade that mask first to invisible and then start actual animation. That way your animated object flashes behind the mask rectangle.
You can set the timings in Timing panel.
