Thank you for posting this in Microsoft Q&A.
You might be getting MFA registration prompt due to Entra registration campaign.
With this feature you can nudge users to set up Microsoft Authenticator during sign-in. Users go through their regular sign-in, perform multifactor authentication as usual, and then get prompted to set up Microsoft Authenticator. You can include or exclude users or groups to control who gets nudged to set up the app. This allows targeted campaigns to move users from less secure authentication methods to Authenticator.
You can also define how many days a user can postpone, or "snooze," the nudge. If a user taps Skip for now to postpone the app setup, they get nudged again on the next MFA attempt after the snooze duration has elapsed. You can decide whether the user can snooze indefinitely or up to three times (after which registration is required).
You can check below article and disable registration campaign in Microsoft Entra portal.
https://learn.microsoft.com/en-us/entra/identity/authentication/how-to-mfa-registration-campaign
Hope this helps in resolving the issue.
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