Wow!
I have used all the 6.5 offerings in Partition and Accubond in 260AI, 6.5x55, 6.5x47 and 264WM.
Not one has ever failed to penetrate, I believe you that it failed to bring it down, I just don't believe it was a bullet problem per se'. More a fact of poor bullet placement, which is not a criticism of anyone's shooting skills.
I have had very few bullet failures, of those I have had, most were with factory 300WM 180gr Power Point loads where the bullet blew up on the hide.
One other notable failure was with a Nosler 260gr .375" pill shot from my 375 Weatherby @ 3000fps at a Water buff here in Australia. The first shot broke the onside shoulder, then the bull spun offering only a hip shot as it departed, the bullet entered with the leg at full stretch right in the hip joint, when it hit the top of the femur, it had hardly expanded and was sticking out of the bone. A third and final shot that went through both lungs and exited was required. I still have both bullets here, they look completely different to each other, the first is perfectly mushroomed like in the Nosler pics in their manuals, the other looks alien with most of the shank intact and the expanded petals broken off. Very weird performance indeed.
I agree, with smaller calibres, a behind the shoulder shot is called for providing a double lung hit in most cases.
Cheers.