Unfortunately I don't have those spells this playthrough. The best I could do was have Gale drink a potion of speed, which added +2 to his AC calculation using Mage Armour on top of the 15 in the screenshot above, for a total of 17. As that's now more than 16, Barkskin correctly ceases to have any effect as per screenshot below. The bug you're reporting, if I'm now understanding correctly, must be specific to the spells you mention or some other factor.

[Linked Image from i.imgur.com]

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"