well, you can upgrade the armor to resist forums of damage (30% resist energy overlay), much more so than any robe. I'm not so sure it's a bug, though: the handmaiden gives it to you, so it's an echani thing, not necessarily a force thing. Also, would armor restrict the amount given by WIS to AC in the same way it does to DEX? I guess I would assume not, since precog has nothing to do with being restricted in armor or w/e.
Summary: not necessarily a bug.