Seems like this is a bug, because I don’t see how it could be useful intended behavior.
I have a piano grand staff. The left hand was actually for electric bass and the right had was for piano. So I eventually added the electric bass player and copied all the material from the left hand.
I then removed the left hand staff. It seems to me that the notes (all MIDI events) should go away when I delete the staff. But instead, not only do they remain, but they are still included in the playback. Worse yet, because you can’t select any of those left hand notes (even with select-all), they do not participate in a mass transpose operation.
It seems to me that at minimum, when deleting a staff that contains material, Dorico should at least offer to delete all that material.
I am able to clean this up by adding the staff, which makes the original notes visible. Then I can delete them.