title: Pandas mode='a', if_sheet_exists='overlay' not working
body: <p>Even using the code below, the content on each sheet of the .xlsx file is overwritten, not appended. What is missing?</p>
  <pre><code>writer = pd.ExcelWriter(excelfilepath, engine='openpyxl', mode='a', if_sheet_exists='overlay')
  
  df1.to_excel(writer, sheet_name='Núcleo de TRIAGEM')
  
  df2.to_excel(writer, sheet_name='Núcleo de FALÊNCIAS')
  
  df3.to_excel(writer, sheet_name='RE - Triagem')
  
  df4.to_excel(writer, sheet_name='RE - Falências')
  
  writer.save()
  </code></pre>