Exactly what it says on the tin, thorn wall can not be summoned onto the lifting platform itself in ‘Halescourge’, it will behave as though it has been cancelled if you aim to spawn it on the platform, otherwise it will spawn at the edges of the platform if you looked over there at any point while holding the button or somewhere on the ground below if you look over the ledge while going down the platform.
I can understand why this might be the case, but considering the ambush of armored rats that waits at the bottom, it was quite inconvenient to suddenly learn this in the middle of a match. 
I went back again solo and this is a consistent issue, thorn wall can seemingly never summon onto the lifting platform, it doesn’t seem to count it as a surface.