Just a quick thing before I reply, the lack of punctuation is making you harder to understand. But I think I know where you're going with this. You're saying that Braums W is most effective when he doesn't use his shield? As the damage taken on by the shield is reduced, and so past that he is taking less damage. This smaller amount of damage then has to go through his Armour and MR, which is boosted by the W. Therefore the damage reduction (through armour and MR) is less than it would have been, for the damage that gets past the shield, had the shield not be there. I think that's where you're going with it anyway.
However, this is assuming that the damage is first filtered through is shield, and then through is resistance stats + W, not the other way around or at the same time. We don't know exactly how its coded. Each of these ways would offer different overall damage reduction.
But really none if this is an issue, as the role of W is not primarily to make Braum tanky. Its to position him between the adc and the enemy, the resistance bonus on top of it is just an extra little thing. And in the end the two together is what makes braum tankier. And besides the shield doesn't do anything against true damage, so it might as well just add resistances.