Yup, it's a bullet failure. The bullet came apart. If you're going to continue using that bullet try hitting them about two ribs back so there won't be so much waste. And I wouldn't use that bullet on elk. IMHO if you really needed penetration, that bullet wouldn't have done much good.
You got him, and I guess that's the main thing. Shots placed about the second or third rib back waste the least meat, though, if you want maximum meat. The hillbillies who taught me about shooting deer were real strict about not wasting meat.
There are plenty of well made hunting bullets that hold together real good and probably won't lose more than 10-20% of their weight.