Have you just installed Cantai?
I encountered the same issue, after quitting and relaunching Dorico the parts (aka the SATB and m/f choir selection) appeared.
The two tracks are caused by Independent Voice Playback and two voices in the “player”.
In Write Mode move everything to one voice and then disable Independent Voice Playback in Play Mode.
But wait…
You have to select the Cantai voice in the Track Inspector rather than in the VST and MIDI tab.
Independent Voice Playback is one of the things you have to turn on to make guitar work. To be honest I never spend any time in the audio magic that lives under control-4 - I’m always terrified I end up with no playback at all.
But now I am here - do you happen to know how I can add a reverb and ambience to the final output - the Cantai voice is very dry .