@0756a23551b406f42794c1dcd0db7379
What happens when they are reintroduced to each other again? Do the relationships restore themselves or do you have to build them back up again? An example of what I mean is when, say, a husband reintroduces himself to his wife, does the profile show them as married or do you have to get them remarried? Do the relationship bars go back to what they were or do you have to do friendly and romantic socials to build them back to what they were?
Since you use mods, MCCC does have an option where you can delete a sim's relationships but it is used on sims one at the time. Afaik, it won't work on a global scale deleting all relationships for every sim in the save. And it would have to be done deliberately by the player. But there may be other mods that I don't know about that can do that. So, if it were me, I'd take a good look at whatever mods you have just to be sure one of them didn't cause whatever has happened so it doesn't happen again.
But it just could have been something has gone wrong with the save or it got corrupted somehow. It's hard to tell.
Hope this helps.