TikZ standalone picture with semi-transparency converted to png looses transparency

Question

I can successfully create PDF figures using TikZ and convert them with convert (ImageMagick) to PNG such that retain a transparent background. However, with the same procedure, for some of my figures, I instead get a white background. It seems that this happens whenever I have semi-transparent elements in the figure (see another SE-question, where this conclusion is implicit in the answer). This is on Mac OS X. If I use Inkscape, I can get a semi-transparent background, but it typically messes up the fonts (and I don't know how to do it via the CLI).

MWE:

\RequirePackage{luatex85}
\documentclass{standalone}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\draw[blue,fill=green,fill opacity=0.1] (0,0) circle (1);
\end{tikzpicture}
\end{document}

The \RequirePackage{luatex85} is there because I use a very recent LuaTeX that causes problems with some packages. Convert with convert -background transparent test.pdf test.png. The result is:

If fill opacity=0.1 is removed, the result looks as follows:

Answer 1

convert calls gs (ghostscript) to convert a PDF file to PNG. gs version 9.10 seems produce a white background if the PDF file contains semi-transparent objects. A recent version of gs (as 9.19 or 9.20) produces the correct result.