Dorico not sending all-notes-off on all MIDI channels?

When I stop playback, my external MIDI keyboard is only receiving an “all-notes-off” message on the channel for the first voice in the score. I.e - whatever MIDI channel is set for the first voice, that’s the one that will receive the all-notes-off.
(just using my ears - I haven’t looked at the MIDI data - it’s possible that I have a config issue with my keyboard, but I can’t see anything obvious)

Oops - I didn’t have the MIDI instrument “activated” - it’s actually working properly now. When it’s not activated, the all-notes-off behaviour is affected by which voice I currently have selected, in the score, during playback.
I don’t understand why the MIDI device works at all though, when it’s not “active”. I’m off to RTFM.

(when I said “voices”, I meant “players”)