Has anyone ever combined Sobel edge post process outline effect with translucent decals?

Sobel edge is working fine. Enable separate transparency is working great for particles. But for some reason, it doesn’t seem to work on decals. Take a look at what’s going on with this cutscene from my game:

No Sobel effect. Smudges on the ground are from translucent decals.

And here is the same scene with the Sobel effect turned on:

There is now an outline around stuff, including the translucent decals. I don’t want that. Notice that the translucent particles are working fine. There’s no outline around or behind those.

I’ve tried every combination on my decal material. Masked doesn’t have an outline, but it also loses all of the subtle opacity. Additive doesn’t have an outline, but it doesn’t give the visual effect I’m going for. I don’t know what modulate does, and I can’t get it to work. Alpha composite has an outline, but in different places. Dithered translucent just looks opaque.

Have any of you ever gotten translucent decals to work with the Sobel edge filter? How did you do it?

I’ll just show you here that separate translucency really is working in general, but just not for the decals:

I put a mesh in front of the pillar. I gave that mesh a material that has opacity 0 and some pretty strong distortion from a noise texture. If Separate translucency were not working at all, then you would see an outline around the mesh (and around the fire too), right?