It didn't entirely block it, no ?
I thought shield on the back had a chance to reduce missile damage by 50%, or something like this ? Anyway, a shield on the back already slows down the poor guy, i don't mind letting him carry one on the back WITH NO FORCEFIELD. If it hits the "shield", it has a chance to be block, else, it's a perfect hit. No forcefield with the shield on the back, only texture's important.