I was looking over at the official thread, and someone named amandachloe found a partial work-around and has a good theory:
Seems i found a temorary solution.
I think that at every thursday morning 2 am, the game creates random new townies, but becuase the world has a limited number of spots, it creates room for the new ones by replacing other sims.
i have been removing random households from the household menu, to free up space by removing people.
seem sthis helps, now the game doesnt have to remove sims, because there is space for new ones.
i have only 1 person dissapearing last 2 ingame weeks
but that was only the first week.
seems i have to remove around 20 sims, maybe 25. for my other sims to be save.
Some people have tested it, with partially successful results--they are losing fewer Sims (maybe one or two). Still not acceptable, but maybe we're getting closer.
It's interesting. I need to make a list of my townies. I have not noticed any of them being replaced, nor new ones coming in, but I may just not be noticing. I have ageing off for townies and ageing on for the active household only, so I wouldn't expect new ones to be coming in, unless they keep populating up to a certain point and then start removing.