A Sim suddenly lost his relationships.

Yeah, I've had Sims forget each other. Playing rotationally, unfortunately, is a risk for that issue. Relationships decay so if you spend enough time away from a household, their relationships will diminish when you get back to them. How fast it decays depends on the relationship.

I really wish Sims would just freeze until you go back to play the household again. MC Command mod has an option to turn off relationship decay. I'm thinking of putting it back into my game.