vtkBalloonWidget prop picking broken

I have a multiple props of which only 1 has had balloon text added using vtkBalloonWidget::AddBalloon().

It does trigger a balloon to show but other props that have not been added with AddBalloon also cause that text balloon to show up.

I've spent quite a bit of time tracking down the culprit and it appears that ultimately, it is vtkViewport::PickPropFrom() that gets called (which makes a virtual call to vtkRenderer::PickProp(). But this vtkViewport method is definitely picking objects (based on screen coordinates) far from anything in the pick-from-list but then it returns a vtkAssemblyPath of an prop far from the pick point.

Interestingly, it picks non-added image slice actors but only sections that are non-black. It also picks other non-added poly data.

I've been able to make a temporary patch by creating a new vtkBalloonWidget class that replaces this->GetAssemblyPath() with this->CurrentRenderer->PickProp() [note: not PickPropFrom which seems to be broken]. This creates the correct functionality but is not a patch to the underlying problem.

VTK 7.0.0 on MacOS Sierra 10.12.3 with cmake/make.