Why might modifying vertex x position in shader.... break z-sort ?

I made a surface shader for sprite , that bends the vertex X position, with vertex Y position as the modifying input.
In the vertex shader, if I read appdata_full v.vertex, and write it straight back out... sprite renders same as normal.
If I modify the x position... x+= someRandomConstant; then write it back out.... so whole sprite is offset in X.... it renders same as normal.
If I calculate some offset..... such as... xAdd = pow (vertex.y + yOff); then add that into vertex.x.... suddenly the sprite starts z-fighting with other sprites and disappears.... but if I move the camera real close... it looks fine.


any idea what I am doing wrong here ? maybe I shouldn't be using surface shader ?
how can modifying X, break Z sort of a sprite ?

Help appreciated.
Thx in advance.