In the following minimal example the HTML comment <!-- (…) --> is falsely not recognized as a comment:

Term
: Def
<!--
: comment def
-->

Pandoc v2.16.2 with pandoc --from markdown --to html5 produces

<dl>
<dt>Term</dt>
<dd>Def &lt;!–
</dd>
<dd>comment def –&gt;
</dd>
</dl>

pandoc --strip-comments produces the same output as above. A workaround is to put any character in front of the commented :. In other words, the commented : (or a ~) is what triggers the bug.