Potentially seeing through walls: a performance issue

[Acies]

I came across one weird performance issue around half a year ago. I don't think I never got it fully answered.

"RE: Water reflection of world
Now I've found the issue. It was quite strange. It has to do with walls I have added on the map. As soon as I remove them the water starts reflecting the world again. But when I add them the water doesn't reflect the world any more. It doesn't matter where I add these walls. I don't know if this is a bug jens, perhaps I should send the map so that you may look at it?

Edit: Worked around it & made the map work."

The walls were 4: wall_corner_concave_round_1 found in the sanctumbase set.

If someone would like to try it out it's map "Area3.map" in the belphegor story.