Is your Shuri duped?
I think what's happening is that the sig allows a chance to perfect block specials. If you don't, you'll take crit fail. Your AA isn't being reduced, you just rolled snake eyes on the PB chance.
I think so too. I was fighting a domino with iron heart in aw and took like 40% of my health in critical failure dmg. And iron heart's kit says she is immune to ability accuracy modification against mutants.