Ideas
This keeps happening to me. I came back to one of the first sims I played with on this save, which is basically my "forever save" after playing her partner's household for a while. Long story short, no matter what, the cow dies. I have Animal Aging turned off, and even fed her a Healthy Treat, resulting in the notification confirming her lifespan was extended. I tried the treat because it happened about 4 times across several different backup save files.
When the treat "worked," I went to the market to get more Lettuce and Lettuce Seeds to make more Healthy Treats. I figured, if the Gameplay Option wouldn't work, I could manually keep her alive. But NOPE. The MOMENT the Sim loaded back into her home, Grim was back. Which is exactly what happened when loading in to other saves, too. Just...immediately. I can't play this anymore knowing that if I do, the animals will die. This pack was supposed to be one of the good ones that won't break your game. :(