Shadow is brightening background image instead of darkening
The shadows are not multiplied over the background image but rather added. So the result is light gray areas where it should be darker.
We’re investigating a few compositing issues with the ground plane and lighting that are causing some artifacts such as the light shadows.
Unfortunately I don't have details on how to work around it. It's a problem with Dimension itself so happens frequently on dark backgrounds, especially with reflective objects or bright lighting and we're updating our compositing methods to prevent the issue (and a few others people have pointed out!)
For the moment I've been darkening the shadows in Photoshop using the .psd export layered render and selecting the shadows with the selection layers.
Any update on this or more insight into what causes it? I seem to be getting this a lot lately.