I saw nothing about this in the latest reports. I narrowed it down to this little snippet. Add a new post, go to the code window, and paste in the code below. It works fine, and if you switch to the Visual window it's still ok, and if you Save or Publish it it's still ok. But if you ever edit it and return to the Code window, the second row of the table vanishes permanantly.
<td colspan="3" align="center">
<h6>This row vanishes</h6>