If you still got one or both "Scroll of Prot Dragon Breath" from CoK or DKK, you might want to check whether they are still unchanged in your inventory.

Once in a while, it happens that they change to "Scroll of Prot" and work like an armour, decreasing AC by 8 when used. Nor more Dragon Breath Protection.

Once, I thought this was because I stored them in the Kristophan vault, but it happened without that too. So far, checking and being careful with savegames is my only protection against this bug.