For example, I add Operator to a track, group it to a rack (Cmd + G) and the Chain Mix Fold Button instantly appears. Is this a bug or have I misunderstood what's printed in the manual?
18.8 Mixing With Racks
Any Instrument or Drum Rack that contains more than one chain can be viewed and mixed alongside the tracks in the Session View’s mixer. A track that contains these Racks will have a 30112.png button in its title bar, which will fold the Rack’s mixer in or out. Likewise, any nested chains within the Rack will also have this button. This makes it easy to get an overview of your Rack’s hierarchy — or hide it when you just want to work on your mix.