UPDATE: They seem to affect the *actual* ac, not just the displayed one, at least at the part where they decrease it.
I toggled them enough to get my AC from 26 (only getting hit on 20s) to 5 and I did get hit by more attacks as the combat log demonstrates. Oddly, an attack total of 10 did *not* hit, but attack that were a bit higher (still way less than 26 though) did hit me.
Either way, if the staff can do nothing about it, I suppose I should destroy this bug incarnate

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//Founder of
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Eldebryn Rilynghym Let the
past crumble to dust. I have the Art... I, Magus...
Mage of Sshamath and teacher of the Art
Gaerdred Dawnfire Paladin of Torm, born and raised in Baldur's Gate