Actually, I was playing around with this a little bit and I think the issue is that when the background color is turned off we use black as the background color. This can cause things to get significantly darker.
Here you can see the background visibility enabled and then disabled. Since we use the background as some information about what your scene should look like it affects the lighting to some extent. I'll review this with the team to see if it's accurate or something we should change, but in the mean time I would recommend rendering with the background enabled then just turning off the background layer in the Photoshop PSD output render. That should give you the correct lighting with bounce light then you can turn the background off to have the transparency.



