Mirror image (and glamour) does not protect units

[Edi]

Actually, the change makes sense. Historically, the way archers were used in battle was that they fired in big, arching volleys (hence the term ARCHery) that targeted the mass of enemy troops. They did not aim for an individual target. So once the shaft is released at the target area, it does not seek to hit anything in particular, nor is it very much impressed by fancy illusions meant to distract the eye. A melee duel is different, of course.