On the left is a gray decal on opaque plastic. On the right is on transparent plastic. Is there a setting I’m missing to make the decal opaque on a transparent material?
There currently is not a setting with which to control that. There is an old YouTrack issue for that RH-29010 Transparency and decals . It isn’t readable to public, but has to do with this issue.
I’ve added this thread to the issue for the record.