To ensure that I understand the issue you're describing, Trunst, do you mean that the effects of an unimproved Devotion Aura and one augmented by talents don't stack, or that Improved Devotion Aura is not granting the additional armour correctly? If it's the former, that is intended functionality -- when two buffs grant the same statistical bonus, the one with the greater effect shall win out. If it's the latter, that is a known display issue with the tooltips for buffs and debuffs improved by talents; essentially, although the improvement is being registered on your character, and even the icon for the spell itself, the buff tooltip won't reflect the change. Rest assured that you're getting the full amount of armour. :)
That's why they call me Mr. Fahrenheit...