Married Sims become unmarried after a few in-game days

I'm playing The Sims 4 on PC with no mods or custom content. My Sims get married successfully, but after about three in-game days they automatically stop being married. They become just a dating couple again, and the "Propose" interaction becomes available as if they had never been married. This has happened three times in the same save, and I never selected the "Divorce" interaction.