Ideas
I have the same problem, no mods or custom content. PC game.
relatively fresh save, only on the first generation, maybe 50 days old. this save was created only about 3 days ago on an up to date version of the game. have not installed or removed any packs since creating the save.
Edith just at some point randomly turned into a townie sim and this townie has all the Edith interactions. I have closed and reloaded the save a couple times, and am unsure of when she changed (if it was during gameplay or on reload). I have noticed the sim wandering around for quite a while (~15-20 days?) but assumed it was just a glitched townie.
I live in the old Ravenwood estate.
I had previously completed the edit/mother questline, and got the mark of Edith etc. and then sometime after that Edith disappeared.
I do not have aging on for any sim outside the active household. I have never played another household than the current one.
I've kept playing and i haven't been able to see what causes it, but edith occasionally turns back inot herself (maybe when i leave the lot and then come back?) but will rather quickly (usually before a day or 2 have passed) she turns back into the same random not-edith sim. edith is not present in the relationship panel while not-edith is visiting.