Are you sure that this is not just a matter of the software you are using to open your png files? See this post: https://forum.corona-renderer.com/index.php?topic=16428.msg133462#msg133462
I'm fairly sure. I regularly save out from the VFB (in 3dsmax) to *.png and open in Photoshop. I'm doing the exact same procedures but from Corona Image Editor and getting solid backgrounds.
(An odd sign is that when I open the *.png files in Photoshop, usually it is listed in the layer manager as a layer (unlocked) by default, but these images are opening in PS with the locked "background" layer automatically...)
What am I missing??? Am I meant to add alpha channel to the render elements or something obvious like this? I find this all very strange. I'll happily share the *.cxr file if you'd like?
EDIT: I should note: I can save out the PNG as desired (ie; no background) ONLY when I'm saving out from the 'Beauty' pass. If I've done some work in the LightMix (which automatically switches the selected/visible pass in the dropdown menu to 'LightMix Interactive') and save it out from there, that's when the problem occurs. Am I to presume from this that I cannot edit the lightmix and still save out to PNG with no background? Or this is a potential bug?