I seem to have something to say about every thing.
That bullet performance is no exception.
Bit of history.
Many moons ago went hunting with some traditional muzzle loader fellas. Both had hand made "long rifles" Kentucky type. One was a 58 cal. Bruised the hell out of every one's shoulder that shot it. Even just once. Mine included.
He thumped a nice muley buck at a bit over 200 yes. Hit it in the neck. That big 58 cal round ball killed the buck deader than a door nail. DRT as we say.
It didn't break the neck. If fact, it didn't break any bone at all. It simply conformed to the curvature of the neck bone. Pure lead is pretty soft stuff. I think softness of the projectile is key but so is hardness.
Next experience is with those HAT bullets from awhile ago. They had very large Aluminum tips with is very long and large diameter shank that set deeply into the 252 grain 338 bullet.
Every one I did terminal testing on followed to some extent the alum tip as it bent instead of coming straight back into the core and opening the jacket. I also "think" the jackets were quite hard.
Thus the tip started to go a direction, the rest of the bullet followed it through the media resulting in a very non-straight wound channel and a banana bullet at it's final resting place.
I'm thinking your bullet followed suit. The petals begin to open and peel off after several inches of penetration. I think about 4" is what I observed in the gel videos I watched.
In your case there wasn't 4" of flesh, or enough flesh for the petals to begin to open. If being made to open make them "softer" than the rest of the bullet, that bullet simply ricocheted off of the vertebrae and took an easier path. Which in this case worked just fine.
I feel that if you would have hit a shoulder blade or rib perpendicular you would have seen a bullet function as design.
You may wish to consider a bit heavier bullet. Ever hear of a 375 SS?
I have been doing some very serious study and evaluation of which CE offering for the 375 AM. Trajectory wise, from 330 to 400 grains there is marginal differences. All have sufficient energy to 2900 yards to kill something.
Thus, it seems, the ability to make POA and POI the same is the deciding factor.
You were able to put POI within an inch or so of POA and that was key. Anything that happened after that is just stuff to talk about.
You're doing an exceptional job.!!!
BTW, did that 700+ yard yote die?