Impervious is the broken trait. Indigestible and Mana Shield, as well as Fortitude, all properly resist their intended effects.
The devs have posted about Impervious being broken; it’s in the 3.1 Known Issues thread.
What they have not done is communicate further on this, insofar as letting people know a fix is coming. Why they do not do so is beyond me. I have seen these traits fixed myself in the beta builds.