MCCC has the option to break up couples. As I know MCCC create a log with changes which the mod makes. I would check the log. Specially if you randomly get the notification its likely that MCCC has something to do with it.
In Vanilla sims can break up but in this case the target sims need to be on lot and talk with the other sim and they need to have a negative relationship.
I have had the same issue and according to the log this is due to the mod. If you want to stop it I believe it is caused by the auto-relationship settings. Sadly this sets me back to square one of trying to keep some sims in my game single ☹️
