Not all nickel alloys are harder than steel. It would seem smart to use something soft, kind of like how they don't use chisel steel for steel cased ammo.
Nickel and its many alloys provide a wide array of benefits to projects spanning multiple industries. In this post we detail the benefits of these metals
continentalsteel.com
For example constantan {55% copper 45% nickel} is right between soft brass and copper in hardness. if you read through this one.
Nickel alloys have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness, and dimensional stability at temperatures much higher than the other aerospace structural materials.
www.nuclear-power.com
What alloy are nickel cases coated with? What did the dies look like before running nickel cases? What was the problem that made you scope the dies?