If I forget to command them to sleep, they will eventually crawl into bed, however they are in an exhausted state at that point and can lead them to be 'unhappy/overworked' and that could lead them to leaving your service. I do not know if this is intended or not. Makes sense if it is.
I mean in the real world if you had a household employee that was hungry, stinky and exhausted, and felt under appreciated, I would imagine they would leave as well.
Suggestion EA - In respect to the sleeping only when they are in the red/exhausted state, maybe a behind the scenes reprogramming to queue them to sleep just prior to entering that exhausted stage or even allowing them rest time (naps for short periods of the day) ? I do not know anything about the programming aspect of it. A warning would be nice ..similar to ones that are given when a toddler is ready to be taken away. Something like -- Butler X is feeling under appreciated, or Butler X is overworked and needs a nap (if they implement such a feature), Butler X is hungry, and then a list of possible resolutions. Butler X may leave your service is these needs are not met for instance. This would clearly without doubt distinguish whether this is an intended game play feature or if you were actually experiencing a game related bug. This may already occur ? I just never let my Butler's needs get out of hand so I have not seen any sort of message.
My Sims also converse with the Butler, play Chess etc.. with the Butler and really make them part of the family or at least I try to include them as such. Even in a simulated game, I dislike treating them as 'hired help'. I try also to pay attention to their emotional state and try to cheer them up etc.. If you praise them (through the Manage Butler interactions), they actually work harder for a bit and get more done in your household. This would lead me to believe that how they are treated could be directly linked to them leaving.
This of course is just my own opinion and I love playing with the Butler and have for years. To be clear my game play experience and opinions/suggestions are not to be taken away from the fact that people may be experiencing actual bugs and technical issues. :)