Merging All Table header cells with Subsequent Cells break the Table Format

Problem

When editing a Confluence table and merging all headers A, B, C with subsequent cells: the table formatting will break.

Environment

Confluence 7.13

Steps to Reproduce

1. Create a new Confluence page with the storage format below

   <p class="auto-cursor-target"><br /></p>
   <table><colgroup><col /><col /><col /></colgroup>
   <tbody>
   <tr>
   <th>A</th>
   <th>B</th>
   <th>C</th></tr>
   <tr>
   <td>1</td>
   <td>2</td>
   <td>3</td></tr>
   <tr>
   <td>4</td>
   <td>5</td>
   <td>6</td></tr></tbody></table>
   <p class="auto-cursor-target"><br /></p>

2. Merge cell A and cell 1
3. Merge cell B and cell 2
4. Merge cell C and cell 3

Expected Results

Cells 4, 5, and 6 should be one row under the new header line cells A1, B2, and C3.

Actual Results

Cells 4, 5, and 6 are on the same row as the previous header line cells A1, B2, and C3.

Workaround

N/A

Notes

Small Recording attached to illustrate the behavior