This happened on one of my legacy saves, but only to one dog (has not happened to cats). The dog was an adopted Toy Spaniel, if it matters.The dog in question seemed particularly needy, constantly needing attention and busting into my sims' queue for praise, and thinking about going for a walk while my sims slept until it passed out. After it died, the family got a new dog and that one has not collapsed (bulldog). That save is heavily modded, though, and I was unable to play it without mods/CC or an entire bedroom will disappear (pufferhead).
I have also seen this occasionally on reddit, but there hasn't been an AHQ report to link to (getting someone to click a Me Too button is hard but easier than asking them to fill out a whole bug report, especially if they can live with the bug).
I'm distracted with IL at the moment, but I can add a dog on my vanilla test save without mods when I have some spare time and can report back. Give me a few days, lots of irl stuff going on.
