The photograph below highlights the problem.
This is the front M Sport brake assembly on the G29 Z4, which is identical to the new G20 3 series with M Sport brakes. The wide open top is like an open mouth and a magnet to attract stones and the bottom of the shield bends inwards towards the brake disc closing the gap and traps the stone. You will also see that the bottom of the shield is positioned hard up against the track rod end and therefore can’t be bent outwards to increase the size of the gap to let the stone escape.
