The 'to-html' function returns;
[CODE=html]
<p><strong>1</strong></p>
<p>
<strong>
<ul>
<li data-xf-list-type=\"ul\">2</li>
</ul>
<p>3
</strong>
</p>
<p><strong></strong></p>
[/CODE]
But the editor renders it as;
[CODE=html]
<p><strong>1</strong></p>
<p><strong> </strong></p>
<ul>
<li>
<strong>
<ul>
<li data-xf-list-type="ul">2</li>
</ul>
</strong>
</li>
</ul>
<p><strong>3</strong></p>
<p><br></p>
[/CODE]
This is the bad HTML which appears to be triggering the issue;
[CODE="html"]<p><strong> ... <p>3</strong></p>[/CODE]
the strong tag is overlapping the <p> and the closing tag is missing
