I have been fortunate enough to shoot quite a few elk in about the last 15-20 years. On average, where I live, I can get two a year. Some years I shoot them with my bow, but I always get a cow with a rifle. I have shot them with numerous kinds of bullets from several different calibers. I think I have come to the conclusion that objects moving that fast shot into something that is not static (like an animal) are just wildly unpredictable. I have recovered a berger VLD 190 out of the far side shoulder of an elk that I probably could have loaded again. Those bullets were famous for exploding inside an animal and being unrecoverable. I have seen ELD-x bullets literally blow up on impact fired from a .300 win mag. I have personally killed at least 6 cow elk with the little 108 ELDm. Of those, I probably recovered 5 that were a perfect mushroom and retained 80% of their weight. I have also seen them hit ribs and "blow up" for lack of a better term.
I have been shooting Hammer Hunters as of late, and I really liked them so far because they were about the only bullet I have shot that produced a substantial blood trail. I thought I had found the perfect bullet and then had a weird instance last year where I caught the front of both lungs and the bullet lodged in the offside shoulder. The elk just kind of laid down and took a while to expire, despite being double lunged technically.
I think some bullets tumble, some expand rapidly (or don't), and some do both depending on the situation. It's all a bit of voodoo to me. I just have focused on shooting behind the shoulder and trying to hit vitals every time. More often than not, that results in a dead critter no matter what projectile I am shooting.