Two causes:
- What I'm seeing on the left (test) round appears to be more akin to a sharp roll crimp than a true taper crimp. What die set are you using ?
- Second, once such a roll crimp starts to bite into the bullet itself (without having a built-in crimp cannelure), it will collapse the case upon further seating. You can see that by the fact that the second (collapsed) bullet itself stopped moving into the case, and a large portion of the shank remains above the case mouth.
For now, seat in one step (no crimp); then unscrew the seating
stem a couple of turns and crimp in a separate step.
The final cartridge should look more like this (ignore that it's a semiwadcutter and just look at the case mouth taper):
