Shader's WorldPositionOffset broken


I’m trying to make a starfield skybox with bilboards. I need to offset stars positions depending on camera position to make it looks like a distant fied.
It works almost perfectly, but thinks happens like if stars are rendered first, then moved. So hidden stars stay invisible, while out of screen stars are invisible to.
I’ve joined two screenshots. I’m using the m_flare_01 material for bilboard’s material.

How do I fix this ?