Yes, thanks for the update. It does seem like the best explanation is a small branch or similar obstruction caused the bullet to tumble. Maybe it was already marginally stable, so when it hit the branch, its stability was completely disrupted and it tumbled like crazy.
My "small branch story": I took a shot at a whitetail deer from a solid rest with a .264 Win Mag. Bullet was 120gn going at 3300 fps. The gun was a sub-MOA gun for 5 shots. The deer was standing broadside to me, placidly feeding. Distance was 200 yards. Just about the easiest shot, ever. The bullet hit the ground right at the deer's front foot. The deer bolted, and, it turns out, never to be seen again by me.
Walking to the spot where the deer had stood to look for blood, a tiny tiny branch hit me in the face about 100 yards from where the deer stood. I didn't give it a second thought. But, recreating the shot later, that branch was right between the shooting stand and where the deer had stood.
Because it was such an easy shot under perfect conditions, I could not believe I did not hit that deer. I spent the next 4 hours, until dark, searching for it. I returned the next morning to search some more, and I had a trained search dog with me. We searched for another couple of hours. We never found any sign of that deer in a 500 yard radius. Not a drop of blood, not a tuft of fur. This dog had an excellent nose and was specifically trained to track wounded game, and if there had been any blood or fur, it would have found it. It was like, . . . the deer had never been hit.
The best conclusion was that tiny branch deflected a 120 bullet going in excess of 3000 fps by about 3' in 100 yards. Not the same as a bullet tumbling, but it shows that a small obstruction can have a big effect on a bullet.