Your bullet being recovered in near perfect condition for the most part I think rules out bullet failure. My initial thought is the tumbling may have been the result flight instability. At 35 yards it is quite possible that the bullets flight had not completely stabilized yet. What leads me to think this is a long time ago a saw a demonstration which involved slow motion filming of the firing of a 30-caliber rifle.
You could see the bullet spinning but what you also saw was a circular yawing of the rear of the bullet. As it turned out the bullets flight did not stabilize until I believe after ~70 yards. Your observation coupled with this may explain what happened. The Woodleigh bullet design uses a Kynoch profile that may also have contributed to it. There is some thought that round nose bullets may have a tendency to be somewhat unstable in tissue. I’m not so sure that this is much of a factor in your case but it’s worth mentioning.