Just like how sydney couldnt use her lighthook on vehemence inside her shield, she cant blast something inside it either. Its some sort of security function most likely, to avoid hurting yourself or anything else being protected. The beam does not shoot invisibly from her ppo then penetrate the shield, it is launched from outside the shield (or possible from the shield wall itself) We just saw her shield is not permeable from the inside as shrapnel bounced around in it and nearly critically injured her.