I contacted Arne, the guy behind NotePerformer. This was his reply…
‘This behaviour happens when “Independent Voice Playback” is enabled. With that option on, Dorico will intentionally create additional channels so it can route separate voices independently.
The key point is that Dorico is fully in charge of the channel assignments and MIDI routing. NotePerformer is a passive MIDI device in this context: it simply loads the channels Dorico requests and plays whatever Dorico sends to them. So we cannot control how Dorico splits channels or how many it creates.
To get back to a normal setup, disable Independent Voice Playback and then reapply the NotePerformer playback template so Dorico rebuilds the routing from scratch”