Ok, this is fixed:
https://github.com/discourse/discourse/commit/a5408650bd693bf643c8d9d976ca56a338f9ca24
While testing I noticed that the autocomplete in the composer for new messages doesn’t show groups unless they are mentionable. That’s probably a sideeffect of Split into two settings: "Who can message and @mention this group?" - #5 by nbianca