Soul hl within a mdframed environment won't print footnote number?

Question

I am trying to highlight some text with soul::hl within a mdframed environment defined by newmdenv. I get expected output using:

• hl text outside of the environment
• normal text within the environment.

However, when I try highlighted text within the environment, the number of the footnote disappears... not sure why?

\documentclass{article}
\usepackage{soul}
\usepackage{mdframed}
\newmdenv[
  topline=false,
  bottomline=false,
]{action}
\begin{document}
This is some normal text\footnote{text}.
\hl{This is some highlighted text\footnote{text}.}
\begin{action}
This is some normal text\footnote{text}.
\end{action}
\begin{action}
\hl{This is some highlighted text\footnote{text}.}
\end{action}
\end{document}

Answer 1

One possible work-around is \soulregister\footnote7 (see also How to make \hl (highlighting) to automatically place incompatible commands in \mbox?)

\documentclass{article}
\usepackage{soul}
\soulregister\footnote7 % <--
\usepackage{mdframed}
\newmdenv[
  topline=false,
  bottomline=false,
]{action}
\begin{document}
\begin{action}
This is some highlighted text\footnote{text}.
\end{action}
\begin{action}
\hl{This is some highlighted text\footnote{text}.}
\end{action}
\end{document}