"Refine Soft Matte" on tracked pre-comp results in black shadow
I'd like to use a soft matte on a pre-comp layer, based on its existing intermediate opacity map; i.e. starting from what's visible as-is, and give that soft edges.
So I thought I could use the "Refine Soft Matte" effect to do just that, but this results in a black shadow around the layer contents, as if they're composed of a foreground pasted on a black background, and the background areas masked out by the mask that gets softened.
The strange thing is, it's still a black shadow when I give the pre-comp a white background, I still get a black shadow.
How do I get rid of that?
The pre-comp uses a tracked 3D layer, but I turned all material options off so I think that should not be the problem.

It feels like this should be solved by clicking one button, but I couldn't for the life of me find it 😅
