Iโd say it looks a bit like rotting, or possibly some sort of pest (like slugs๐ก) eating away at it. If it is slugs, look for a slight shiny look on the others
The soil looks way to wet and heavy. Dry all the roots out thoroughly. Repot in fresh soil. Add perlite for drainage. Don't water immediately after or for at least 2 weeks after the repot.