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As I understand you have disabled the security defaults on tenant and also have made sure that per user MFA is disabled for all users in Azure portal. Still users are getting prompted for MFA.
You can check the sign-in logs in Azure AD for that particular user who is getting prompted for MFA and check which feature is triggering for MFA.
It might be triggered due to Identity protection as well.
Let me know if you have any further questions on this.
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