First table row is always saved as header row

Follow these steps to reproduce:

• edit a new page
• insert a table
• fill the table cells with some values
• the first table row is not a header row by default; you can even open the menu for the first row and see that "Header row" is not checked
• save

Expected result: the first row is not a header row
Actual result: the first row is a header row

If you edit again and open the menu for the first row you'll see that "Header row" is now checked. Uncheck. The first row becomes a regular row. Save. The first row is again a header row.