Hello,
You’re creating a second actor with coincident geometry to render the selection. So, there is likely z-fighting or depth-fighting going on between the model and the selection actors. Please, take a look at this: Visualization failed when display two polygon in the same time - Support - VTK . Particularly, see this post: Visualization failed when display two polygon in the same time - #9 by zhang-qiang-github (map2 in that post is a vtkMapper).
best,
PC