Annotation -> Markups: disable line shading?

Lines in the annotation module were rendered flat, i.e. without shading.

The lines from the Markups module are now shaded by default. Is there a possibility to configure flat shading for these? I have searched, but currently (in 5.8.1) do not see a way to access the properties of the actor used for rendering the vtkTubeFilter output from a module.