The specify voicing in the manage parts isn't behaving as it should. It seems to be when you have two notes in a single layer and two notes in different layers in the same measure.
In general, this feature has a lot of potential, but is lacking. I often have movements where I want part of it to show single note passages and part of to not to. I end up having to cut corners and put things in multiple layers for the parts when the score doesn't need it.