I' ve never before heard that the Nosler Partician was designed to lose the foreward cavity lead.
The front portion of the partition is designed to give you the rapid expansion you need for quick kills and to provide a large wound channel (often all that is left of the front is the jagged cutting edges of the jacket), with the added benefit of the base holding together and providing additional penetration (usually about 60% of the starting bullet weight). Best of both worlds expansion and penetration.
Well, maybe the Nosler Partition isn' t exactly DESIGNED to lose the front core, but that is often what happens! The two I' ve found inside an animal, after great penetration and finally hitting a big joint which stopped the bullet, were missing the front cores. Damage in one case was unbelievable (everything ahead of the diaphragm was a bloody soup, with no piece of lungs or other organs larger than a quarter), and the remains of the front portion of the jacket were folded back, constituting the " mushroom" .