[upstream] CKEditor 5 mangles table structure

Problem/Motivation

This table structure cannot be saved when using CKEditor 5:

<table>
  <tr>
    <th>Header</th>
    <th>Header</th>
    <th>Header</th>
  </tr>
  <tr>
    <th>Header</th>
    <td>Data</td>
    <td>Data</td>
  </tr>
</table>

Steps to reproduce

Paste code above into the editor source view, then either save or toggle back out of source view. All of the <td> tags will be converted to <th> tags.

There seems to be an upstream bug here: https://github.com/ckeditor/ckeditor5/issues/11536 https://github.com/ckeditor/ckeditor5/issues/3172

However this happens regardless of any tbody structure or not.

Working examples

headings in <thead>, first column headings inside <tbody>:

<table>
  <thead>
    <tr>
      <th>Heading</th>
      <th>Heading</th>
      <th>Heading</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>Heading</th>
      <td>Data</td>
      <td>Data</td>
    </tr>
    <tr>
      <th>Heading</th>
      <td>Data</td>
      <td>Data</td>
    </tr>
  </tbody>
</table>

colspan heading in <thead>, first column headings inside <tbody>:

<table>
  <thead>
  <tr>
    <th colspan="2">Heading</th>
    <th>Heading</th>
  </tr>
  </thead>
  <tbody>
  <tr>
    <th>Heading</th>
    <td>Data</td>
    <td>Data</td>
  </tr>
  <tr>
    <th>Heading</th>
    <td>Data</td>
    <td>Data</td>
  </tr>
  </tbody>
</table>

Breaking examples

(paste these into source, toggle out of source or save the content and all <td> will be converted to <th>)

<table>
  <thead>
  <tr>
    <th>Heading</th>
    <th>Heading</th>
    <th>Heading</th>
  </tr>
  </thead>
  <tbody>
  <tr>
    <th colspan="3">Heading</th>
  </tr>
  <tr>
    <td>Data</td>
    <td>Data</td>
    <td>Data</td>
  </tr>
  </tbody>
</table>

<table>
  <tbody>
  <tr>
    <th colspan="3">Heading</th>
  </tr>
  <tr>
    <td>Data</td>
    <td>Data</td>
    <td>Data</td>
  </tr>
  </tbody>
</table>

This one flips the <th> to a <td>

<table>
  <tbody>
  <tr>
    <td>Data</td>
    <td>Data</td>
    <th>Heading</th>
  </tr>
  </tbody>
</table>

After trying to narrow all of these down, it seems like the behavior is "a <th> may only exist as a non-colspan first column inside a <tbody>"—any other structure results in tag data loss.

Edit: It's also not allowing <td> inside a <thead> at all, and that's also valid HTML.