Some of our clients are joined to Azure AD on their laptops running Windows 10 using Single Sign-On.
(Settings > User Accounts > Work or School Account > Connect > Join to Azure AD directory instead > Office 365 email address)
Azure AD sign-in on their laptops was running fine unless they ran windows update on their system and restarted their system. The Azure AD account disappeared completely from their sign-in option on the laptop. We had to use the local account (backup account) to sign in to their laptop, restore Windows to a previous build in order to fix the issue.