Bump! Experiencing the same issue, sim received moodlet "Unfair Affair" after being exposed in a cheating scandal. The original moodlet was 9 days, same in the notebook.
Next day my sim travelled and the moodlet changed to a different higher number around ~16. The tooltip in the notebook stays correct. Throughout the course of the game, the number kept changing, especially triggered by travelling it seems. There is a third indicator I noticed much later on, which is when you hover over the sim and see the count there too. That one just does its own thing entirely; it's currently at over 300 days, but hit over 1.5k at some point. The moodlet itself remains smaller; still hit something like 80 days at some point.
The moodlet itself is fixed if changing the sim in the mirror, but does go awry when the sim moves anywhere.
I have mods and I am not ready to get rid of them at the moment, though they are minimal, but on the sims reddit I saw quite a few people having the same issue, some confirming without mods. Posting this to raise awareness of the issue