In reading your reply, I"m wondering if you’ve used an Expression Map? I don’t know the VST you mention, but I’d check to see if any notes a “locked out” because those keys (notes) have been reserved for the Map. Usually Expression Maps are assigned to Keys way outside the usable range of the instrument, keys without assigned samples or sounds.
Not sure, but you may find it useful to Dissolve the Part by Pitch. Or, you can assign a Voice to each line… This is more complex, but look at that. In this case it may help. Cubase will also generate harmony voices and if you have a chord track, the harmonies are based on the chords.
IMHO, Cubase’s Chords, Scales, Vocings and Styles are pretty darn good. Not perfect, but good.
Sadly, there are some bugs too. For example, if you place chords on a chord track, add a transpose track and a transposition point (or points), when playing back the sequence of chords in a loop, the second time the loop plays, the chords do not Transpose. The Transposition will work if you have MIDI on the tracks and are not using the Chord Track to playback the chords.
BUT, if you want to experiment with different chords and just use the Chord Track for playback, Cubase won’t play the Transposition in Loop on the Second Pass. It works if you stop and re-start, but in Loop, the chords do not transpose. IMHO, this is a bug. Anyway, that’s really off topic, but here’s a screen shot to illustrate. So, if you start to work with a Chord Track and Transpose Track BOLO for this issue.
Good luck with your work.