[Might be an older problem, but I ran into this using LuaLaTeX in TeX Live 2026 with June 2026 updates applied.]
\documentclass{article}
%\usepackage{unicode-math}
\begin{document}
Consider $X_\mathcal{R}$.
\end{document}breaks when unicode-math is enabled, requiring the author to write $X_{\mathcal{R}}$. On the other hand, with $X_\mathbf{R}$, it works with or without unicode-math.
(I was helping an author who wanted to recompile a previously-working (non-LuaLaTeX) document in a modern environment to comply with current accessibility requirements, thus we switched to LuaLaTeX and made some other preamble changes which—among other things—pulled in unicode-math.)
[Might be an older problem, but I ran into this using LuaLaTeX in TeX Live 2026 with June 2026 updates applied.]
breaks when
unicode-mathis enabled, requiring the author to write$X_{\mathcal{R}}$. On the other hand, with$X_\mathbf{R}$, it works with or withoutunicode-math.(I was helping an author who wanted to recompile a previously-working (non-LuaLaTeX) document in a modern environment to comply with current accessibility requirements, thus we switched to LuaLaTeX and made some other preamble changes which—among other things—pulled in
unicode-math.)