I think I've figured out why the Ghost Saber will die after taming for no reason. To get the cat, you have to open the figurines, which summons the cat through the game mechanic. The hunter then tames the saber, but there's probably a spawn, wait, despawn mechanic on the mob. So, at the time where the cat would have usualy despawned, he dies. However, because he's now your pet, you can res him. Both myself and a friend got these recently just for fun, and they both died after a couple minutes of being tamed. Since then, they haven't died. I think after you get past the spawn, despawn mechanic, you own the cat like any other pet.