This is because the malformed table content added manually in this edit gets "refostered" by Tidy in read mode but not in edit mode; the plan, in T89331: Replace HTML4 Tidy in MW parser with an equivalent HTML5 based tool, is to remove Tidy and replace it with something more modern, and exposed in such a way that these issues get fixed correctly by users rather than brushed under the rug (or broken in unexpected ways).

In this case, this is T109897: Table parsing diffs: Parsoid adds implicit <td>s after a |- if explicit pipe is not present.