Welcome to the forum, cwhyles. On the face of it this doesn’t look like a bug: there’s no specific fixed order to the way Dorico handles voices, beyond there being one or more up-stem voices and one or more down-stem voices, so Dorico uses a sophisticated set of processors to determine the optimal arrangement of notes and rests when there are two or more voices active. Dorico doesn’t think that it would be clear to show that rest in the middle of the staff, so it moves it down below the staff. You can override this by setting the Rest pos. property for that rest, which will allow you to move it vertically. I hope this helps!