I’m not at my computer just now, so please confirm that you’re talking about the NotePerformer mixer, not the Dorico mixer. NotePerformer only ever shows one channel in the Dorico mixer (or possibly one channel per instance, if there are multiple NotePerformer instances).
Note that you can use the new NotePerformer (SO) playback template provided by @David_Tee last month to have one instance of NotePerformer per instrument, so you can really use Dorico’s mixer properly.
I’ve used it already in quite some of my scores and am very happy with the ability to control everything from Dorico’s mixer.
I know it’s been about 8 or 9 mo. since the last posting here but I just got the NP4 update and now the Dorico mixer decides to show all the channels that I am using but only the instances in Dorico mixer can be edited. How do I get it back to just having the one instance (or so many) of NP showing up in the Dorico mixer instead of all the channels of all the instances? It really clogs up the area with this happening. Thanks so much in advance;) sort of looks like this