If you find yourself here because this issue is affecting you, try looking in your save folder.
I noticed that the ver1, ver0, etc files each had many, many duplicates. Something like:
Slot_00000002.save.ver0
Slot_00000002.save 2.ver0
Slot_00000002.save 3.ver0
Slot_00000002.save 4.ver0
Slot_00000002.save 5.ver0
Slot_00000002.save.ver1
Slot_00000002.save 2.ver1
Slot_00000002.save 3.ver1
Slot_00000002.save 4.ver1
If you see that, then you've probably got iCloud Drive's Desktop & Documents feature enabled; I suggest turning it off. I don't get the impression that it works particularly well with the Sims. I found mods in the mod folder that I thought I'd deleted and, pertinent to this issue, I believe the above issue is a symptom of the sims having difficulty saving the game, perhaps because the files it's trying to read aren't locally available (synced to iCloud and not downloaded).
Turn off the feature, and move your sims folder back to the local Documents folder manually.
