I've had this happen twice over the course of the last week. My current butler is hard at work, and I'm playing on my home lot. A new sim arrives in their butler uniform. I notice the original butler no longer has the Butler title, but the new one does. When I go to assign a bed to the butler, the butler available is the new one. After a bit, the original butler says it's time to go and leaves, and the new one is on the job now.

I've returned home from traveling to find a new butler, but I've never seen one replace another like this when I've taken no action to initiate it. There's no alert that you're butler's being replaced or anything. They just swap out and the old one goes home.

I've confirmed with LMS that this is not a new feature of the Better Butler mod. I don't know of any of my other mods that would affect the butler like this, so it's either a bug, or a new undocumented feature.