@haisin The 20th is when this all began for me because I did not place pets in my Eco save when the pack came out. I was busy playing around with the new 'features' and was too occupied to deal with fur babies. Since this bug appeared, most of my playing time has been spent experimenting with different settings on the pets, the weather and generally trying anything and everything that might at least lessen this bug's impact and cut down on the number of times the pets are snatched out of the Sim houses. And they are almost always disappearing while interacting with the owner, the food dish or while obsessing over toilets and tubs. As @Kita5399 observed in her game, her dog was barking at the tub in a bathroom that had auto lights enabled, and after the dog was snatched off the lot, that bathroom light did not turn off. The game thought someone was in the bathroom and did not turn the light off until the dog re-entered the lot. The game believed the dog was still in that room even though the dog was actually not even on the lot. Which is very odd.
Something similar has been happening with the plants that might be tied into the weather and seasonal effects that were updated for EL in the pre-ep update we got on June 3rd. Players began reporting the issues with the plants and the pets as soon as June 4th. Then the pack was released on the 5th, which didn't leave the teams any time to fix those bugs before we made the mistake of buying it. But even without owning the pack, the gardening features and the pets were glitched from the moment we installed the update.
@RavenSpit When the cat with the prowler trait goes off to wander, does it come home on it's own, or does it need to be summoned? Has it disappeared during the day? I believe the prowler trait pushes the cats to go out only at night and occasionally they bring back feathers or gifts for their owners. Has the cat returned home with either of those? What I find interesting about your experiences, is you said you have never scolded either pet. This is the interaction I thought might be the reason why the pets began to disappear in the first place. Everything was fine in my game until I lectured the dog for barking all the time, and the cat for waking it's owner in the middle of the night, and for scratching the furniture. Training the dog to do tricks doesn't seem to cause any issues, but the scolding or lecturing does. At least, that is how things happened in my game.
My thanks to both of you for responding. Any little bit of info you may have could be relevant so please let us know how you're doing, and what you have observed your pets doing. :)