Hi,
Yes, it’s your HTML. You cannot add a <tr> like this as it is not appended to the table directly, but added in a <td> within another <tr>.
This means you end up with invalid HTML and the browser will try to fix it – resulting in this strange behavior.
I’m not sure what you want to achieve here, but if you add an “empty” heading, this is what the HTML looks like:
<tr id="heading_66" style="display: table-row; cursor: default;" fieldtype="dffs_heading"><td class="tabH1" colspan="2"><div></div></td></tr>
where “66” is the heading ID I added in the “Unique ID” field for the heading.
Alexander