Circuit library: Omega symbol not showing up in the resistor label when using package fontspec

[voen-void]

Brief outline of the bug

I copied the code from https://tikz.dev/library-circuits#sec-49.1.1 to try out the circuit library.
I intended to use the circuit library in my thesis, in which I need to use the fontspec package to set a special font on the title page of the document (requirement of my uni). Upon compiling, I noticed the Omega symbol missing in the resistor label in the schematic. Compiler I used is XeLaTeX, though the same happens in LuaLaTeX. After some investigation I found out that this behaviour only appears if the package fontspec is loaded -- when I remove it, the symbol shows up normally. I also found out that if I import the package fontspec with the option [no-math] the Omega symbol shows up normally in schematic labels. In either case I'm able to use math typesetting, and $4\ \Omega$ works as expected. I'm not sure if I'm missing something, or if this is a bug in the package.

Minimal working example (MWE)

\documentclass{article}

\usepackage{fontspec} % this is the trouble maker
%\usepackage[no-math]{fontspec} % this works

\usepackage{tikz}
\usetikzlibrary{circuits.ee.IEC}

\begin{document}

Math typesetting test: $4\ \Omega$.

\begin{tikzpicture}[circuit ee IEC]
    \draw (0, 1) to [resistor={ohm=4}] (2, 1);
\end{tikzpicture}

\end{document}

[hmenke]

Not a MWE. See I've just been asked to write a minimal example, what is that?

[voen-void]

Not a MWE. See I've just been asked to write a minimal example, what is that?

I updated the 'MWE' -- I hope it complies with the requirements now.

[hmenke]

Compare

$4 \Omega$\par
$\mathrm{4 \Omega}$\par
$\mathrm{4 Ω}$

To work around this use

\tikzset{circuit declare unit={ohm}{Ω}}

or (as you have already discovered)

\usepackage[no-math]{fontspec}

[hmenke]

See also josephwright/siunitx#435 for an in-depth discussion why this happens.

[voen-void]

Thank you for the reply and workaround -- it works! The thread you mentioned is very informative, I was surprised that this issue goes so deep.