I can't confirm it with that particular item, but I'm seeing identical behavior with "Unwavering Resolve", a necklace. Intellect and Resolve, stats the necklace have, have since increased Durance's permanent stats by 5 or 6 points.
It also appears to be happening with Kana's Turban, which increases Intellect by 1. His base Intellect is 17, but even without the hat, it's now at 20.
So it seems this bug possibly affects every character with various items. I still have no idea what directly causes the increase. Saving and loading at certain points, I assume, from reading other posts about it.
