Just some additional info:
The game mechanics allow for mean autonomous socials as soon as sims have the broken up relationship bit. There are no additional tests so in a vanilla game, Sims that have once broken-up will always be able to autonomously be mean to each other no matter how strong their relationship got afterwards.
That's why so many experience this bug now since the scenarios are a thing - most players don't tend to break couples up and bring them together afterwards unless encouraged to do so by a scenario or other challenge.
