Many times in late game, I use Lux's ult to clear minion waves because the cool down is so low. When the enemy minions meet up with my minions and begin to bunch up and attack, I will ult them. When they are bunched up like this, I have noticed many times that even though a minion is well within my laser beam, he will be unharmed AND my passive won't activate on that one minion. For some reason, when bunched up one minion will be immune to the damage and the passive. So, the whole wave will be demolished, except for the one.
There's my report.