When I click Mute or Solo in the Mixer, all the tracks/instruments respond. If I click “S,” all tracks are Soloed except the Drums, which is now muted. If I click “M,” all tracks are muted, except the Drums. That track is unaffected.
Is there a video (of a recent version of Dorico where the windows look like V.6) or an area of the manual explaining this behavior?
This sounds as if you are using NotePerformer or another VST that is sending all its output into a single channel, so when you mute or solo, all channels are affected.
Some VST’s (including the latest versions of NP) allow one to assign each instrument in the VST mixer to a different channel in the Dorico Mixer.
Otherwise do your muting and soloing in the VST mixer rather than the Dorico mixer.