It’s possible that their centers do have the same Y position – I can’t tell for sure, though.
I suppose if there are one or two (or maybe even three) nodes in the group, you may want to do your own positioning rather than depend on CircularLayout’s positioning behavior. One way to implement that would be to override CircularLayout.commitNodes to see if there are one or two vertexes in the CircularLayout.network and then adjust their positions before calling the super method.