This likely isn't the cause for all issues related to this but one potential cause I have noticed:
I have two sims who had a bitter about breakup sentiment a long time ago (as Teens and they are Elders now) and it is fully gone from their relationship profile now, but they still have the option to "ask what their problem is" in the mean category as if the negative sentiment were still active
I feel like it's not gone away properly (similar to how the crush sentiment never seems to fully go away even when it's gone from the sentiment panel unless your teen "gets over" it) so it's causing the sims to be mean to each other
The Sims Team