I agree the above situation is not fun, and should be avoided.
First point - the players at the table should define terrain. Ht 2 and 3 terrain that is less than 100mm in diameter probably shouldn't get to be stood on.
And if it exists, then you shouldn't really send ht 2 models within 2" of it.
(I'd argue it is pretty realistic, if someone is standing on something above my head, I can't see them to hit them with a sword if they want to be avoided, but they can easily get themselves to be able to see me when they want to. )
I view the above situation as a situation that the rules allow, but the players shouldn't. Just like ht 8 sniper towers or 24"long river pieces that count as 1 piece, or spaces only incorporeal models can reach.
So whilst the above situation is allowed in the rules, its the most extreme situation possible of this instance, and to get to it, the players need to rule the terrain can do that.
I don't see this as an argument that non reciprocal LOS is a problem, but rather models shouldn't be allowed to stand on terrain that no one else can stand on that is just the right height to screw up a crew (Make this a ht 3 rock and it gets a lot less of an issue in the above example, (but causes the same issue if you are facing lots of ht 3) since she can no longer bleed on them, and make it ht 4 and Nekima can no longer hit ht 2 people even though she can see them).
I can certainly make a bunch of other situations that would lead to a similar outcome of Nekima is engaging a group of models that can't engage her back without it being a reciprocal LOS issue. But most of them would be caused by me deliberately using the game rules to create such a situation.