Are you sure? It makes sense that the damage reduction wouldn't take effect until the damage actually hits Paladin, which it doesn't with Stoneskin.
Why would Defiance have no effect on Stoneskin otherwise? The HP value is a static value. Buffed Stoneskin is just 18% of Max HP even when combined with Thrill of Battle. What would be a reasonable way to test that theory? Gather the average damage of a normal mob without Shield Oath and then compare it to how many hits it takes to remove Stoneskin. Even if you apply Defensive buffs + Shield Oath if the amount to remove Stoneskin is greater than the calculated shield value then CD's and Shield Oath does effect Damage pre-Paladin Damage.