Hello,
Welcome to Microsoft Community.
Can you log in to Microsoft official website or other Microsoft products to receive 6-digit security code through the identity authenticator?
Because 8-digit security codes are usually applicable to work/school accounts, it is recommended that you test to see if the current reinstalled authentication is tied to the correct Microsoft account.
If there is anything unclear, please feel free to let us know, thank you for your patience and understanding.
Regards,
Manson |Microsoft Community Support Specialist