It shows both Altos as being the same (default) voice.
In the meantime I’ve found something out that might be interesting for the Dorico team:
If I paste the reduction into a staff that has had only one voice (or was ever completely empty), the reduction is perfect.
In my case the target staff has had 2 voices: The first one had been used for the Alto 1, and a second one that had been created when I did “Paste into new voice > new downstem voice” with the Alto 2 notes. As I mentioned in the top post, this staff was used to hold a combination of Alto 1 + Alto 2.
What I did before I went to test the reduction feature: I selected everything that had already been there and pressed .
It seems that somehow this second (downstem) voice was kept active, so when Dorico pasted the reduction into this staff, the notes from Alto 1 were put into this still active downstem voice, and the notes from Alto 2 were put into a new upstem voice.
I’m a bit confused, because after my “delete everything” action I did not even think of the possibility of voices being active or not.
I’m not sure that the current behaviour is expected behaviour in any way. Either Dorico should reset voices when I “delete everything”, or Dorico should start with voice 0 when doing the reduction. Or at least give me some kind of warning. I don’t know. It just feels wrong in a way…