I have seen this phenomenon. In my experience, it has been an issue of the kid "guessing" at the pitch instead of actually tracking it with his eyes. Solved it with soft-toss from the side, using 2 balls thrown from the same hand. Just as I'm releasing, I tell him which one to hit (top or bottom, as balls are stacked in my hand). I've also used the very small balls that have different color numbers on them. Batter has to call out the number or color before swinging. I think that works better, but those things really fly, and could get through a net.
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