Icons with elements that have opacity different from 100 show black backgrounds in KDE apps
Mind you opacity is not the same as alpha channel, This problem occurs by setting the opacity.
You can test this behavior using gwenview.
Example icon you can test by yourself:
Use the example icon from this commit specifically as I later fixed this issue by using the alpha channel.
Related issue: https://gitlab.gnome.org/GabMus/giara/-/issues/61