@Paigeisin5 when everything still worked as intended, the cat would always come back on his own, but he did prowl during days and nights (sometimes I made the sim send him out though) and sometimes he'd bring the owner gifts, yes.
But now, the cat doesn't really run off, he just disappears (it says "null" under his name, when I hover over the portrait) and I can't bring him back at all, selecting the "call for" icon does absolutely nothing (meaning it's not even queueing the action).
I am gonna check on my other households with pets now and see if they disappear.