"Mechanics are beyond justification. This is a board game; those are the rules and nothing else matters."
Mechanics without justification are an arbitrary set of rules that are, in worst case, impossible to remember because of their arbitrariness.
Hint: Those are bad mechanics. You don't want those in games based on themes and atmosphere.
This game, trying to fill the gap between board game and role playing game (which it does, look up every review. No I don't care about your personal opinion, you can keep that one, but don't force it on others), has to offer an understandable approach to rules.
Although I can deal with the fact that lamps cancel blindness in MoM, I do think that it's a matter to discuss.
If you don't think so, you're free to go. We know it doesn't bother you. That's alright. Just don't annoy those who want to talk about it.
Now, after feeding the troll, back to the topic on hand:
Lantern: Investigators may ignore darkness in your room.
Loss of Vision: Whenever you explore, treat the room as if it were in darkness.
The wording, in my opinion, is clear: Lanterns cancel the effect of darkness, that is caused by exploring while having "Loss of Vision". However, "Loss of Vision" doesn't appear to be "blindness" or anything like blindness.
Look what happens: If an investigator explores, he gets a disadvantage. During combat, though, s/he is not affected at all by the trauma.
Yeah, meta-gaming says this is due to balancing issues.
But in-game I'd expect "Loss of Vision", if it really is the loss of someone's eyesight, to affect combat like darkness does.
Imho, "Loss of Vision" is... well... did you ever search for a pen that you had in your hand? Or the glasses you just placed on your desk, so you had a hand free to open a drawer to search for your glasses inside? I certainly did, and I expect the Trauma Card to represent a much more severe case of this.