I haven't really played much with butlers, because none of my families has been rich enough to afford one, but this also happened to me months ago while I was playing.
I had made a butler for a particular family and hired him. But I think after leaving the lot for something and travelling back, the butler had been replaced with a newly generated one.
The sim living in that household is in the scientist career, so it's possible that after going to the active work lot and returning, the game had replaced the butler for some reason, but I can't really remember because I immediately exited the game without saving and reloaded an earlier save to keep my desired butler.
I'm not really sure how the game designates butlers. Maybe it doesn't save a particular butler to a particular household, maybe the game just considers any butler NPC to fill the role?
I was hoping to make a second household with another butler, but was worried they'd keep swapping butlers between them or spawning completely new ones.
Or maybe it was just a random occurrance? I don't know. The game seems to keep consistent mailmen, maids and gardeners for particular households... like I have two gardener NPCs in my game, but when I hire one of them it's always that one that comes and not randomly the other one on some days.
