in 5.x eZ has changed the way it assign headers inside a table. It's now assigning the depth of the section to the header's inside of table, no matter what level you assigned in the editor.

test:

• create new object, disable OE editor
• insert in any xml block:

  <header level="1">Heading 1</header>
  <header level="2">Heading 2</header>
  <table border="1" width="100%">
  <tr>
  <td>
  <header level="1">Heading 1</header>
  <header level="2">Heading 2</header>
  </td>
  </tr>
  </table>
  

it will show result:

<h2>Heading 1</h2>
<h3>Heading 2</h3>
<table class="renderedtable" border="1" cellpadding="2" cellspacing="0" width="100%">
<tr>
<td valign="top"> 
<h4>Heading 1</h4> 
<h5>Heading 2</h5> 
</td>
</tr>
</table>

there was an old issue where this was fixed, but it appears that tag configuration with XSL in symfony has reverted the behavior