Explain the problem.
Not all parsers support Groff style \[xx] escape sequences. Eg. on macOS, they cause issues with whatis:

$ cat pandoc.html
<h1>NAME</h1>
pandoc &mdash; general markup converter
$ mkdir -p man/man1
$ pandoc -tman pandoc.html > man/man1/pandoc.1
$ /usr/libexec/makewhatis man
$ cat man/whatis
pandoc(1)                - [em] general markup converter

I would suggest using the standard \(xx style escape sequences, eg. \(em which are supported by all parsers.

This should apply to other two-character escape sequences in --ascii mode as well, eg. Δ becomes \(*D, which has been around since troff was introduced.

Longer escape sequences can use \C'nnn', eg. \C'hbar'.

Pandoc version?
pandoc 3.6.2, macOS 15.3.2