21 days ago - Sleipnir - Direct link
Sounds like everything is set up properly and that's definitely not intended for them to stop auto-joining. The people that have decided to live alone, do they have companions?

Either way, it sounds like a bug. Does re-loading solve anything? I'd be happy to look at the save file too if you'd like to send it my way.

One behavior I've noticed that can happen is people stop mating because the entire city becomes one family. I'll be adding some things to help avoid this in the future.