Could it be possible that youwere loading with too heavy of a charge?

Threeyears ago I shot a buck in the right shoulder with a 570 round ball. We trailed the deer for over 250 yards and I shot the deer again in the back of the neck. My meat cutter recovered the bullet and told me that he found it just under the right shoulder. It had flattened to the diameter of a quarter and about twice as thick. I had noticed that the exit wound in the deer's neck was a split much like a knife blade wound. Eight days earlier I had a simular experience with a doe but she went right down but no exit wound.

I was shooting 130 grains of Goex 3-f with an average velocity of more than 1800 fps. Since that time I have backed down to 90gr of 3-f for an average speed of 1600 fps. I have since killed eight deer with the 90gr load and got complete penetration and an exit wound every time.

This made me wonder if you might be pushing the bullet too fast causing it to fragment.