"Can't see you" and "you are outside its line of sight" are the same thing for me?
You said "can't see you
from inside the icewall" as if the ice wall has something to do with it. (It doesn't.)
And it's definitely different for bosses and normal monster. Normal monsters definitely see you from inside icewall. Despite them permanently dropping target, they will keep aggro on you as long as you stand near the icewall. If you walk around the icewall they will even leave it to the south/west.
For boss monsters it seems different. They will just not move.
They are different with respect to how they behave when the cell that they occupy is non-walkable, as I explained in my first post. In addition to what I said, bosses also can't move from that non-walkable cell. However none of this is relevant to the issues of range and line of sight.
Usually the "rudeattacked" skills are supposed to prevent such exploits, but with that icewall trick by standing next to the MVP (even though it won't even attack you!), the rudeattacked check seems to fail and it won't teleport away. Seems quite exploitable to me.
Attacks from within a monster's range are not rude attacks, as anyone who has ever used unchained serenade on Cecil Damon can attest. I suggest that you try both this strategy and the mindless brute force strategy before passing judgment.
I'm actually curious why it's different for boss monsters. Is it because boss monsters can't see through icewall while normal monsters can? That's my current theory on it.
Again, the differences are not important. In my Ifrit video, Ifrit could not see the attacker because there was a (real, solid, part of the map--not ice) wall in the way. Any monster in that position, boss or otherwise, would be affected in exactly the same way.