About the time I woke up I remembered the test Box o' Truth did on some British Mark VII ball ammo, in particular that with a wood insert in the nose. Looked it back up, and there's a definite similarity there.
Looking at the Sierra bullets, the design puts a lot of weight toward the back with a big empty space in front of the core, much like the wood insert moves a lot of the weight back in the ball ammo. So I think the Sierra acted much like the ball: instead of expanding(at this velocity, at least) it went unstable very quickly and deformed badly. Which, if I'm correct, would give a good result in the event of needing to use it in self-defense.
More thought needed.