You will think I'm picking on you, but I honestly am not sure of what you describe. As shielder, you get behind a polearm guy, and because he does a overhead, you get stunned with the backswing ? I seldom experienced that, and anyway the ability to backswing is more of a problem for polearm users because they hit teammates all the time with it.
Anyway polearms need some fixes, right now with turning you can hit people all around you with a side swing no matter the angle, your position or theirs. Hence why a majority of polearmers just dance around barely noticing the enemies.
It happens whether you have a shield or not, but without a shield you can at least instantly block when they turn and R/L swing at you. With the shield delay, you simply don't have time after recovering from the stun effect. It is too slow. Even so, they shouldn't be getting a free stun by cocking their weapon in an overhead. (the backswing)
Typically you get behind them, prepare to slash their backside and whoops, you are stunned by their overhead swing being cocked back when they are facing away from you.
I believe it used to interrupt
them, so they complained and now it stuns whoever is behind them, yes teammates included.