Guardian stance does not apply deflection bonus to Eder.
Here is my Eder without shield having 34 deflection.
With the shield (34 + 12 from shield + 6 from shield specialization) = 52 so far so good.
Here during the fight with guardian stance enabled. It is still 52 also in combat log. Please note it shows on active effects -10 acc + 10 deflection. This bonus is not applied however. (Penalty of acc is applied).
Deflection bonus does work on my other party member. It just doesn't work for Eder (source of aura).