Ideas
Might be more reasons they get replaced but my sims traveling/ going somewhere and coming back home is the most obvious one.
Recently I had my household go on vacation to Granite Falls. When selecting sims to go with me I could chose the butler, now I figured he might just as well join since he is basically part of the family anyway.
Well, I got another butler joining me for the vacation. This is just one example of traveling and the butler getting replaced, most of the time it is just about going to the park, or visiting another sim etc and when my sim gets back home the butler is missing and another one is there instead.
Butlers getting replaced has been an issue since Vintage Glamour came out, but I have noticed it not happening every time I travel now (then again, I don't use butlers much because of them getting replaced, so I might just not have had enough of them).
I will create a separate report for butlers not feeding babies and infants.
I play on pc with no mods or cc