Hi Iniobong Nkanga,
Thank you for posting your query on Microsoft Q&A!
If you have disabled both Per-user MFA and Security Defaults then the source of MFA requirement would most likely be Conditional Access.
If you navigate here do you see any enabled Conditional Access policies - https://portal.azure.com/#view/Microsoft_AAD_ConditionalAccess/ConditionalAccessBlade/~/Policies
Also you can check the sign-in logs for a user who recently was required to perform MFA it should should which Conditional Access policies applied.
One other possible cause is that users are a guest in another tenant, and when accessing apps like Teams or SharePoint they are required to perform MFA due to Conditional Access in the guest tenant.
Do let me know if this helps or if you have any further queries.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.
Kind Regards,
Donal