It’s really not when we have Belo'vir as a soul shard who wasn’t even killed by arthas and instead just lost an arm and then was slain by undead.
Like losing An arm is enough to split a soul but having the blade plunged into your head and bring you into undeath isn’t? It’s not like she didn’t have a soul for frostmourn to scar and rather the wound is mortal or not apparently has no baring.