No that wasn't bullet failure.
I would say you probably witnessed near the limits of that particular bullet's performance parameters. In otherwords, you might not want to use that bullet for headshots on a cape buffalo (I'm assuming you shot an American Bison).
I don't think the designers of the bullet you used had 100 yard bison headshots with .300 Win Mag in mind when they were sketching up the details. You pushed its limit and it did not fail.
Basically, a hunting bullet failure is when you failed to recover or would have failed to recover the animal because of the way the bullet performed. Sometimes the bullet splatters and the animal runs off with one less leg but no damage to it's vitals. Sometimes the bullet doesn't mushroom and the animal runs so far away you lose it.
There is no such thing as an idiot-proof bullet. Although modern technology is closing the gap. Idiots expect the same performance regardless of shot distance, shot placement, impact velocity, and game animal. The wise shooter will understand that he must change his behavior when the variables change.
