fpeval expands past its argument about latex2e

[u-fischer]

That has nothing to do with \fpeval but is the standard behaviour if you use low-level TeX counter assignments. It continues to look for more numbers and expands commands until it hits a space (or something else that stops the parsing like \relax). See the difference here:

\RequirePackage{latexbug}       % <--should be always the first line (see CONTRIBUTING)!
\documentclass{article}
\begin{document}
\newcount\x
\x=2
\x=1%
\ifnum\x=1 ok\else error\fi

\x=1 %
\ifnum\x=1 ok\else error\fi
  
\end{document}

[domenjoud]

Yes!!! You are completely right! I missed this.