Office 365 makes extensive use of Azure Active Directory guest accounts. Implementing a risky sign-in policy is a good idea, but it can have the unfortunate side-effect of suddenly blocking guest accounts that could previously access tenant resources. If blocks happen, they can only be lifted through administrative intervention in the guest account’s home tenant.
https://office365itpros.com/2019/09/25/aad-risky-sign-in-policy-blocks-guest-access-office-365-tenan...
