standalone package wrongly claims sub-preamble of a subfile changed if used with \file_input:n instead of \input

Question

As shown by the following MCE, the standalone package loaded with its subpreambles option gives the expected result with \input, but not with \file_input:n when the input file has a non empty preamble. In such a case, the content’s of subfile2.tex isn't inserted and following warning is emitted:

Package standalone Warning: Sub-preamble of file 'test.tex' has changed. Content will be ignored. Please rerun LaTeX! on input line 7.

But the sub-preamble obviously didn't change. Do you understand what happens?

Edit

New MCE in order to clarify my question.

\begin{filecontents}[overwrite]{subfile1.tex}
\documentclass{article}
\begin{document}
Content's~of~\texttt{subfile1.tex}.\par
\end{document}
\end{filecontents}

\begin{filecontents}[overwrite]{subfile2.tex}
\documentclass{article}
\newcommand{\foo}{}
\begin{document}
Content's~of~\texttt{subfile2.tex}.\par
\end{document}
\end{filecontents}

\begin{filecontents*}[overwrite]{myclass.cls}
\ProvidesExplClass
  {myclass}
  {2023-01-03}
  {0.1}
  {
    My Nice Class
  }
\NeedsTeXFormat{LaTeX2e}
\LoadClass { article }
%
\RequirePackage[subpreambles]{standalone}
%
\NewDocumentCommand{\myinput}{ m }
{
  \input{#1}
}
\NewDocumentCommand{\myfileinput}{ m }
{
  \file_input:n{#1}
}
\end{filecontents*}

\documentclass{myclass}
\begin{document}
\myinput{subfile1}
\myinput{subfile2}
\myfileinput{subfile1}
\myfileinput{subfile2}
\end{document}

Original MCE:

\begin{filecontents}[overwrite]{subfile1.tex}
\documentclass{article}
\begin{document}
Content's~of~\texttt{subfile1.tex}.\par
\end{document}
\end{filecontents}

\begin{filecontents}[overwrite]{subfile2.tex}
\documentclass{article}
\newcommand{\foo}{}
\begin{document}
Content's~of~\texttt{subfile2.tex}.\par
\end{document}
\end{filecontents}

\documentclass{article}
\usepackage[subpreambles]{standalone}
\begin{document}
\input{subfile1}
\input{subfile2}
\ExplSyntaxOn
\file_input:n{subfile1}
\file_input:n{subfile2}
\ExplSyntaxOff
\end{document}