Problem with font in theorem environment

Question

I use LuaLaTeX, fontspec and XITS maths font. Additionally, I had a problem with theorem environment and long headers so I based my solution on this. My problem is that I now get a different font for the \mathcal{S} when I use optional name. Any ideas why?

\documentclass{article}
\usepackage{fontspec}
\usepackage{amsmath}
\usepackage[math-style=ISO]{unicode-math}
\setmathfont{XITS Math}
\setmathfont[range={\mathcal,\mathbfcal},StylisticSet=1]{XITS Math} 

\usepackage[thmmarks,amsmath,amsthm,hyperref]{ntheorem}
\makeatletter
\newtheoremstyle{Myplain}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2.\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2]\theorem@headerfont (##3).\theorem@separator\normalfont\itshape}
\newtheoremstyle{nonumberMyplain}%
  {\item[\theorem@headerfont\hskip\labelsep ##1\theorem@separator]}%
  {\item[\theorem@headerfont\hskip \labelsep ##1]\theorem@headerfont (##3).\theorem@separator\newline\normalfont\itshape}
\makeatother

\theoremstyle{Myplain}

\usepackage{thmtools}
\declaretheorem[numberwithin=section]{theorem}

\begin{document}

\section{Title}
\begin{theorem}[A a a very very very very long long name theorem theorem name name]
A named theorem $\mathcal{S}$.
\end{theorem}

\begin{theorem}
Unnamed theorem $\mathcal{S}$.
\end{theorem}

\end{document}

Answer 1

It seems that ntheorem issues a \mathversion{bold} command for the head, that you don't protect with a group. This is evident if you say $\mathcal{S}=a+b$ in the theorem's body.

Just add braces to the style definition when you use \theorem@headerfont.

\documentclass{article}
\usepackage{fontspec}
\usepackage{amsmath}
\usepackage[math-style=ISO]{unicode-math}
\setmathfont{XITS Math}
\setmathfont[range={\mathcal,\mathbfcal},StylisticSet=1]{XITS Math} 

\usepackage[thmmarks,amsmath,amsthm,hyperref]{ntheorem}

\makeatletter
\newtheoremstyle{Myplain}%
  {\item[\hskip\labelsep {\theorem@headerfont ##1\ ##2.}\theorem@separator]}%
  {\item[\hskip\labelsep {\theorem@headerfont ##1\ ##2}]%
         {\theorem@headerfont (##3).}\theorem@separator
          \normalfont\itshape}
\newtheoremstyle{nonumberMyplain}%
  {\item[{\theorem@headerfont\hskip\labelsep ##1}\theorem@separator]}%
  {\item[{\theorem@headerfont\hskip \labelsep ##1}]{\theorem@headerfont (##3).}%
         \theorem@separator\newline\normalfont\itshape}
\makeatother

\usepackage{thmtools}

\declaretheorem[
  style=Myplain,
  numberwithin=section,
]{theorem}

\begin{document}

\section{Title}
\begin{theorem}[A a a very very very very long long name theorem theorem name name]
A named theorem $\mathcal{S}=a+b$.
\end{theorem}

\begin{theorem}
Unnamed theorem $\mathcal{S}$.
\end{theorem}

\end{document}