Ideas
I came back to a household I hadn't played in a while and immediately my couple who are engaged and who I left happy are unsatisfied in their relationship. Then they autonomously started a fight during a happy conversation. I built their relationship back up from a full red friendship bar and it happened a second time and ruined their relationship again.
This has been happening to me too. No NAP's that include fighting, not fight day, relationships with compatible ppl who are attracted to one and otherwise happy - just start arguing and doing mean interactions and then physically fighting until they divorce. No matter how much I focus on getting them back on track - they fight again! I don't use any mods or CC but have all of the EPs.
I've had the excessive arguing happen before but it built up more gradually over time and was easy enough to fix if I spent time focusing on the relationship again for a bit. But it's gotten seriously out of control lately.
I spend two full days getting my most recent couple back up to least not red friendship bar and then I took my eye off them for a second, physically fighting, all the way red again! It feels futile to play any couples or relationships at this point.