# Fontspec redefines Siunitx symbols

I'm trying to find a solution to an already described problem here. The \Omega symbol does not display when loading fontspec (in this case implicitly from the package polyglossia). I've found this problem but I'm sure it will affect other symbols.

I think page 58 from the siunitx manual dedicates some words to this, but I don't know how to interpret/fix the problem. Here is a sample:

\documentclass[12pt]{article}

%% Language of the document
\usepackage{polyglossia}
\setdefaultlanguage{spanish}

%% Units
\usepackage{siunitx}

%% Change document's fonts
\usepackage[charter]{mathdesign}

\begin{document}
$R=50\si{\ohm}$\par % Does not work
$R=50\Omega$        % Works
\end{document}


It seems that the T1 font encoding that mathdesign package applies to the document does not like to be with XeLaTeX, so better use an OpenType font with math support like stix2.

\documentclass[12pt]{article}

%% Math and text fonts
\usepackage{stix2}

%% Units
\usepackage{siunitx}

\begin{document}
$R=50\si{\ohm}$\par % Works
$R=50\Omega$        % Works
\end{document}