Home realm discovery during sign-in for Microsoft 365 services
We are changing our Azure Active Directory (Azure AD) sign-in behavior to make room for new authentication methods and improve usability. During sign-in, Azure AD determines where a user needs to authenticate. Azure AD makes intelligent decisions by reading organization and user settings for the username entered on the sign-in page. This is a step towards a password-free future that enables additional credentials like FIDO 2.0.
Source: Azure Roadmap