Thank you for posting your issue on Microsoft Q&A.
I understand that you are receiving eight-digit authenticator code when trying to sign into personal Microsoft Fabric account. Even though it’s an 8-digit code.
In Microsoft accounts we get 6-digit code in authenticator app while in personal accounts we get 8-digit code.
Try to Re-register MFA. As you are global administrator you can Re-register MFA for user.
When you sign in again, you will be prompted to set up your MFA. Follow the prompts to add the Microsoft Authenticator app again and sign in account you will receive code.
Hope this helps. Do let us know if you have any further queries.
If this answers your query, do click `Accept Answer`
and `Yes`
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Thanks,
B. Siri Chandana.