Odd. I run with a Heavy steed, so I haven't noticed any differences myself. But I know a bunch of other Cappies do use Lights, so maybe they can chime in. I'd think they would have mentioned it by now, though.
Initial thought: Are you in a group when you see this happen? Could be that Stun Immunity (which could be triggered by LMs applying their mounted stun, for instances) also affects Dismounts?
[b][SIZE=3][COLOR="#FFFFFF"]Cainwen Ciaphas[/COLOR][/SIZE][/b][COLOR="#C3C3C3"], Captain of Crickhollow, [url=http://tvtropes.org/pmwiki/pmwiki.php/Literature/CiaphasCain]HERO OF MIDDLE EARTH[/url]!!!
[i]I will not be pushed, filed, stamped, indexed, briefed, debriefed, or numbered![/i] My opinions are my own![/color]