vtkDelaunay2D documentation describes the limitations of the filter in great detail, including potential failure in case of non-convex shapes and lack of points, so I don’t think the filter could be considered buggy.
The main problem I see is that vtkContourTriangulator was not mentioned in the “See also” section of vtkDelaunay2D documentation, so VTK users may not be aware that a better alternative exists. I’ve submitted a merge request that adds it: https://gitlab.kitware.com/vtk/vtk/-/merge_requests/11669