I had the girlfriend of my sim's son (not actively playing but living in a neighborhood house) just randomly move out of their house with the grandkids. The really weird thing, the son stayed living in the house. The GF, her sisters and the kids, gone.
Edit. Happened again today with another offspring family in my game. This time I was paying more attention to about the time it happened. It happened right around the time the grandkids aged up to teenager. So looks like some kind of bug when they age up outside of the active household. Wonder if it is another bug related to the "beloved" aspiration trait as my sim has that.