My dog and cat also vanish. Even if I have the camera on them, they literally just disappear. The portrait panels are grayed out, and if I hover over them, it often says they are at Journey's End, which is in a completely different world than the one I'm playing! So, my pets spontaneously vanish and go to another world in the game. This happens even if I have the doors locked for pets. It happens even after putting a fence around the entire lot and locking the gates.
It happens every few minutes. I have to use the phone to call them to invite them to "hang out" on the lot where they actually live, and even that does no good. I get a message saying they'll come right over, and then they come back and then vanish again within a few minutes. I created the pets when I created the household. They are not adopted strays. I have full relationship bars with both, the backgrounds of their portrait panels are always green, and one is at companion status in the game...not that it matters, since there's no way to keep them on the lot. They don't have any traits that would cause this - neither are adventurous or prowlers or anything of the sort.
If I'd known the pets expansion functioned this way (as in, it's essentially unplayable) I wouldn't have purchased it. You don't actually get to have pets with whatever glitch this is. All you get is grayed out portrait panels of pets who are technically part of your household, but who only come to visit for a few minutes if you call them on the phone and spend 99% of the time in other worlds. Honestly, if they weren't sims of my pets in real life that I created for my simself, I'd probably just adopt them out and be done with it. They might as well not even be members of my household when the game works this way. It truly does make the game unplayable because they vanish every few minutes, sometimes even while my sim is interacting with them. I've been a Sims player since Sims 2. The pets expansions have always been must haves for me, so to have the pets expansion function this way is very upsetting. ☹️
I have no CC and no mods in my game. The game and all of the expansions I have were downloaded directly from EA via Origin.