I've had two 168 Barnes fail and was present at the failure of a 165 Barnes from 300 WBY and 30-06, failure was deflection on heavy elk shoulder bone and they did not penetrate the rib cage. All elk were recovered and it's no guess as to failure!!
The Barnes and Ballistic tip are the two bullets I hate to see guys show up with to kill elk, I drank the light, fast, hard bullet coolaid but got tired of elk flopping around or running away with a leg doing the helicopter. Went to softer bullets and I have not had a deflection since even when trying to make it happen nothing but dead elk!!!
What I think is happening is the bullets frontal area vs mass is just not enough, but a lead core bullet will shear of some of it's frontal area which removes bone from the path and gets the frontal area down, and allows it to keep driving into the vitals. If you shoot at a gong at 500 yards you'll hear the Barnes deflecting into space and a lead core bullet you'll find little copper disks under the target from the bullet staying on path until it expends all it's momentum. I also can stabilize much heavier bullets with a lead core so I gain momentum over the lighter copper bullet, which I feel is more important than hitting them with ultra fast bullets.