I have noticed negative impact on friendship with some of my SIMS as well. I got all members of the household in one room when the negative impact happened, it did not effect any of my other SIMS in the household (There was no other sim in the household with a negative impact or a disintegrating friendship bar) So I tested it in another household. Under "mean" interactions there is an option to "trash another sim" I did on one Sim, trashing the same Sim to other SIM toons. When i got the two from other households together, they disliked one another.
This is about random signs over the sims head that have nothing to do with what's actually happening. You might be seeing a different issue.
Are you using mods ?
