According to asciidoc doc:

When text is enclosed in a pair of single or double hash symbols (#), and no role is assigned to it, the text will be rendered as highlighted (aka marked) text for notation purposes.
https://docs.asciidoctor.org/asciidoc/latest/text/highlight/

BUT Pandoc prepends a role [.mark] which breaks the highlighing when converted to HTML:

> pandoc -t asciidoc

[test]{.mark} text.

[.mark]#test# text.

The output should just be #test# text.

https://pandoc.org/MANUAL.html#highlighting