Per:
https://github.com/discourse/discourse/commit/7a9eee1b71cf298210ee9a258e9f17ae1a8a9af7
All groups except the “moderators” group have default tracking state of watched, in admin groups you can customize this to whatever.
Note, the default applies when people are added to the group, to apply retroactively simplest thing to do is add/remove user from group, or visit user profile page and change it there.