This is very interesting too. Something else we have learned that I would have bet against. We have figured out that a lighter bullet that sheds the nose will out penetrate a heavier bullet that sheds no weight. This goes against the math that says the heavier projectile will have more momentum which would result in traveling farther through the animal or test media. This also comes down to stability. The heavier projectile that is carrying more momentum is also longer. Because it is longer it needs more rpm's to keep it point on. As it loses rpm's it begins to turn sideways. Now it greatly increased the surface area that the media is reacting to and causes the bullet to stop faster resulting in less penetration. The bullet that sheds it's nose winds up with a shorter retained shank that will maintain stability longer while paying through the media resulting in greater distance of penetration. If too much mass is lost on impact then this would result in less penetration. So there is a line where this is no longer true.
If there is less contact surface, there will be less deflection and alteration of its course, especially if the frontal area remains rounded and evenly shaped, like if it sheds all its petals and is left only with a shank remaining. If only one petal remained, that remaining petal would cause the bullet to alter its course, sometimes drastically- same as if the whole bullet started turning and tumbling without expansion.
Tipping in flight doesn’t just come from reduced RPMs, it comes from the center of pressure sliding back to or behind the center of gravity of the bullet, which causes the nose to tip and allow the bullet to begin to tumble.
Shedding weight will allow the bullet to also shed momentum and velocity and that in turn will increase the penetration up to the point in can no longer overcome the resistance it is encountering. This is why starting with a good deal of sectional density (mass) is ideal with a bullet that sheds weight. You want enough of the bullet remaining to continue to penetrate. A heavier bullet that does not expand but also does not penetrate deeply typically does so because it tumbled upon impact and increased its contact surface to the point its forward momentum is indeed arrested and it stops penetrating. Coming apart and reducing contact surface would allow for more/deeper penetration.