As far as I remember (unless stabilization policy changed), stable packages must not involve unstable package versions. There is no even stabilization requests for any >=pugixml=1.11 . Reproducible: Always
You’re correct that it’s policy and violating that would break the tree’s consistency. But vtk is an unstable only package...
Oh, my appologies. I just checked it at gpo.zugaina.org, and indeed all versions are keyworded with ~* keywords. Just can't find, what makes it stable.