[Python] Pandas mode='a', if_sheet_exists='overlay' not working

Even using the code below, the content on each sheet of the .xlsx file is overwritten, not appended. What is missing?

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()

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