Vertex color emptied after instanciating mesh at runtime (build only)

I have a project where I'm using vertex colours as data to procedurally stretch my meshes, ie red goes left, green: forward, blue: up, I don't use them to render at all. At run-time I load the mesh and modify the vertices using the colour information. T$$anonymous$$s works perfectly in debug, but in a built version the color data in the instanciated meshes is empty.

If I manually place a the meshes in the scene with a material that uses vertex colours t$$anonymous$$s seems to force unity to know it has colours and it works fine. However t$$anonymous$$s is an ugly hack, is there somet$$anonymous$$ng I'm missing?