Please use this thread going forward: Sims forget already existing relationships | EA Forums - 11894834
I also just recently developed this problem since the most recent patches (June, July). I started a sim in July who lives in San Myshuno and went to university, so she meets new sims regularly. But she keeps forgetting them rather quickly, even the ones she lived in the dorm with (so, talked to every day). I tried to create a Get Together club for the teammates she played e-sports with at college, but she didn't remember some of them anymore so now I wouldn't know how to reintroduce her to them. She makes friends with neighbors in San Myshuno and days later forgets them. I saw some comments mention culling but I've pretty much only played this household for the past couple (real-life) weeks and she forgets people within a few Sim days. Why would the game be culling Sims that quickly? It's not like she's forgotten an NPC she met weeks ago in real life time. She meets a sim on a Monday in the game and then forgets them by Wednesday when she runs into them again at the food stall.
I do have mods but I'd be happy to backup my save files and test without if that would help. But it looks like others without mods are having the exact same problem.
