It’s getting really late in my time zone and I’m not by my workstation anymore, but I can see if I can recreate something that shows the effect first thing in the morning for you! Unfortunately I can’t show the asset I’m working on so it would need to be something different.
I believe you have misunderstood my setup though. I am not using a PNG in the scene, instead I want to render out a png with transparency from the movie render queue.
So what I want is my main asset in the render, with a transparent background. This works, but as soon as I add a post process material to my post process volume what becomes transparent before in the rendered image is now black. Does that make it clearer? I apologize for any confusion!
As I said I think this is because the post processing acts as a layer that’s added on top of the rendered image and, while it doesn’t affect the image, it does remove any alpha layer from the underlying render.