The id attribute is dropped from headings when converting from HTML

The following HTML:

<h1 id="one" class="two">
  <span>Heading</span>
</h1>
<p id="three">Paragraph</p>

gets converted to:

(% class="two" %)
= heading =

(% id="three" %)
Paragraph

The id attribute is dropped from the heading. This happens for any level of heading. I suspect the reason is because the rendering thinks the id was generated.