So it looks like the system image libraries are really messing things up and not handling alpha correctly at all on 10.15 for PSD files. I swear it wasn’t like this in previous OS releases.
I’ll have to file a radar on this or find some interesting workaround. It does look like premultiplication is getting in the way, big time- but I’m not able to undo it because the transparent areas, which are black technically, are getting baked into the image.
One thing you can do though, is use the latest build of Retrobatch which actually has an unpremultiply node in it. If you use it twice in a row (ie- have two nodes of it), it will clean up the gray, but kind of wash out the transparent areas a little bit. It’ll also get rid of the drop shadow unfortunately.
What I’ll need to do to fix this is write my own composite reader for the PSD images, but that’s not something I can just whip up. I will look into it though.