I am not 100% sure what is going on here, but it is not at all unusual for games to have a max limit on resistances. Most likely they just put a resistance cap at 80% so you can't go completely immune to damage without using a skill or power. IIRC that was the same in DA2 and Origin as well.
I had a similar suspicion, but I still think it is a bug. If the intent is not permit 100% resistances, then that should be documented and more importantly, rather than letting people discover this after burning tons of resources, the game resources should be structured in such a way that you can't build 100% resistance. The bug may be that Snoufleur Skin is overpowered.
