that's the way partitions are designed, halfway back inside the bullet is a copper "H-Mantle" fused to the jacket, the rear core is harder lead protected by the h-mantle, the front of the bullet is a soft pure lead core that as the jacket peels back is desined to come apart as the harder rear core holds its shape and continues to penetrate.
If your running a high intensity cartridge you may be better served by the accubond, its the same bullet as the ballistic tip except the jacket is fluxed, just exactly like a plumber fluxes a copper pipe so the solder sticks to it, the accubond may become a twisted mass of lead and copper but it stays togather and keeps penetrating, they normaly expand to +2x diameter and retain at least 60% of the original weight even on high velocity impacts.
RR