Sounds like you had a bullet failure.

Bullets for the .45-70 generally are generally designed to expand well, even at modest (or SLOW) velocities. Compared to a similarly designed bullet say in a 30cal, a .458" bullet is often built more "lightly" so it expands under lower speeds, than the 30cal bullet would.

So the bullet basically popped like a balloon upon impact, giving the large entry wound, and poor penetration. (think about doing a cannonball into the pool as a kid, that rebounding "spla-doosh" tall splash is what happened to the surface of your deer's wound).

At a longer range, or a slightly slower velocity, this might not have happened. I have the same issue with my .30-06 and CT Ballistic Silvertips. Under 50yrds, they're just moving too fast, and I've had 2 bullet failures in 10yrs (similar to yours). On the other hand, at 300-600yrds, these bullets are absolutely FANTASTIC.