Passing sourceItem-based texture to postprocessing effect uses the dummy texture

I am trying to render a 3D scene into a texture so I can add it to another scene through post effect. But the input texture in the effect's fragment shader is transparent.

You can find a minimal example attached. The final rendering is supposed to be 2 black spheres (one at left, the other at right) with a red background

The right one is rendered on the first view3d with a transparent background and is added to the second view3d (which render the left one and the red background) with a effect