It looks like this no longer works as I described earlier.
I don’t think we ever really explicitly designed it to work as it was earlier. It’s possible it was kind of working by accident before and regressed after recent changes since there are likely no specs that cover this behavior.
I’ll move this topic to Contribute > Feature to more explicitly record that this is something people would like to be able to do.
Users will have to add it themselves currently – they can click “Reset to defaults” on the sidebar categories modal after they are in their appropriate groups, but unfortunately, they still need to take some action themselves until something else is done here.