Steps to reproduce :

• insert a table, for example a 3x3 table
• set the left column to be the "column header" (the left column now use <th> instead of <td>)
• from any cell of the table, press the "insert a line above" or "insert a line below"
  The whole table become "headers" (using <th> instead of <td>).

This can be seen more easily if the table is using the dark style (<th> have a background color)