# Minus-plus symbol disappears inside \raisebox{}{}

I am attempting to raise an \mp minus-plus symbol slightly to match an existing typeset equation. The problem is that the \mp symbol disappears when I put it inside a \raisebox{}{}. It works fine when I use a \pm symbol. I only have this one instance so I thought \raisebox would be a quick and easy fix. If I put the \mp inside an \fbox it doesn't look like there is any character inside.

The existing equation is illustrated here:

My code:

\documentclass[12pt]{article}
\usepackage[
math-style=french
]{unicode-math}
\setmathfont{Latin Modern Math}
\begin{document}
\begin{center}
Why this $\mp$ disappears?
$PH = {\raisebox{1pt}{\mp}} \frac{1}{\sqrt{2}}$
But this $\pm$ does not?
$PH = {\raisebox{1pt}{\pm}} \frac{1}{\sqrt{2}}$
\end{center}
\end{document}

produces:

\raisebox is like \mbox and takes you out of math, so in traditional tex, both commands would have given errors, the fix is the same, go back in to math with $\mp$ and $\pm$.