This appears to be a bug in mathastext relative to its "subdued" mode. Brief explanation: mathastext reassings mathcodes of letters and digits and some ascii characters. When "subdued" mode was added, this was done not by reverting the mathcode changes (the package does not keep memory of earlier situation) but by hacking into the LaTeX math versions in order for the Font to be used to match the original, un-modified, fonts for either letters or operators.
For some reason I have forgotten, when copying over the pristine "letters" and "operators" status to the mathastext own math groups, mathastext deliberately modifies the font encoding to be the \encodingdefault (at time of loading mathastext). In the MWE where mathastext is loaded after fontspec, this encoding default will be TU, hence, rather than storing \T1/futs/m/n (as set up by fourier package for digits) in its own digits font for subdued mode, it stores \TU/futs/m/n. This then causes:
LaTeX Font Warning: Font shape `TU/futs/m/n' undefined
(Font) using `TU/lmr/m/n' instead on input line 37.
which explains the altered looks of digits in math mode.
In the case where mathastext is loaded before fontspec, and after fourier, the \encodingdefault is the T1, not TU and all goes well.
I have forgotten the rationale behind this usage of \encodingdefault. There is also some code to deliberately replace OT1 by T1 if \encodingdefault is OT1. All of this was done at a stage of developement of mathastext when it had never been tested with xelatex/lualatex/fontspec anyhow.
One could imagine for the OP's problem modifying \encodingdefault back from TU to T1 for the duration of loading mathastext after fontspec but from brief look at code this would give unexpected consequences due to some other usages of \encodingdefault in the package code.
Of course, one can also fix this by re-doing the mathastext hacks, but this is a bit scary code. It would look something like this:
\documentclass{article}
\usepackage{fourier}
\usepackage[no-math]{fontspec}
\setmainfont{erewhon}
\usepackage[subdued,italic]{mathastext}
\makeatletter
\def\mst@reserved#1\getanddefine@fonts\symletters#2#3\@nil{%
\def\mst@normalmv@letter{#2}}
\expandafter\mst@reserved\mv@normal\@nil
\def\mst@reserved#1\getanddefine@fonts\symletters#2#3\@nil{%
\def\mst@boldmv@letter{#2}}
\expandafter\mst@reserved\mv@bold\@nil
\def\mst@reserved#1\getanddefine@fonts\symoperators#2#3\@nil{%
\def\mst@normalmv@operator{#2}}
\expandafter\mst@reserved\mv@normal\@nil
\def\mst@reserved#1\getanddefine@fonts\symoperators#2#3\@nil{%
\def\mst@boldmv@operator{#2}}
\expandafter\mst@reserved\mv@bold\@nil
%%
% \edef\mst@tmp{\encodingdefault} % NO!
\def\mst@reserved#1/#2/#3/#4/{\gdef\mst@debut{#1}\gdef\mst@reste{#2/#3/#4}}
\begingroup\escapechar\m@ne
% \xdef\mst@funnyoti{\expandafter\string\csname OT1\endcsname}
\expandafter\expandafter\expandafter
\mst@reserved\expandafter\string\mst@normalmv@operator/
\endgroup
% \ifx\mst@debut\mst@funnyoti\ifx\mst@tmp\mst@oti\def\mst@tmp{T1}\fi\fi
\edef\mst@normalmv@operator{\expandafter\noexpand\csname
%\mst@tmp/\mst@reste\endcsname}
\mst@debut/\mst@reste\endcsname}
\begingroup\escapechar\m@ne
\expandafter\expandafter\expandafter
\mst@reserved\expandafter\string\mst@boldmv@operator/
\endgroup
% \ifx\mst@debut\mst@funnyoti\ifx\mst@tmp\mst@oti\def\mst@tmp{T1}\fi\fi
\edef\mst@boldmv@operator{\expandafter\noexpand\csname
%\mst@tmp/\mst@reste\endcsname}
\mst@debut/\mst@reste\endcsname}
% \AtEndOfPackage{
\typeout{** ...correcting subdued mode...}
\expandafter\SetSymbolFont@ \expandafter\mv@normal\mst@normalmv@letter\symmtletterfont
\expandafter\SetSymbolFont@ \expandafter\mv@bold\mst@boldmv@letter\symmtletterfont
\expandafter\SetSymbolFont@ \expandafter\mv@normal\mst@normalmv@operator\symmtoperatorfont
\expandafter\SetSymbolFont@ \expandafter\mv@bold\mst@boldmv@operator\symmtoperatorfont
\typeout{** ...done.}
% }
\makeatother
\setsansfont{TeX Gyre Heros}
\renewcommand\familydefault\sfdefault
\Mathastext[sans]
\renewcommand\familydefault\rmdefault
\begin{document}
123 $123$
% \textsf{123 \MTversion{sans}{$123$}}
\MTversion{sans}
123 $123$
\MTversion{normal}
\thispagestyle{empty}\showoutput
\end{document}
Of course it is not practical to add such a big hack to preamble (I could reduce its size but kept close to original, just to modify it here and there). Hopefully some update to mathastext will fix this, but the author will have to remember why in first place the code insisted on using \encodingdefault (there must be a reason...).
1.3t. – Sep 08 '18 at 08:07