Scene Capture 2D additive not working for FinalColor (HDR)

I’m using scene capture 2D additive to generate two images: one with RGB Inv Opacity in A for background crop out; one with Final Color (HDR) for rendering the actual color.

After capturing the scene, I noticed RGB Inv Opacity in A respects the additive flag and render out as I expected; However, Final Color (HDR) will clear out the previous image and always render the current scene - is this a bug? Or am I missing something?