We have the same problem that the GPO "Block Signing into Office" (requires installation of Admin Templates btw: https://www.microsoft.com/en-us/download/confirmation.aspx?id=49030), which translates to the registry entry "signinoptions" seems to be ignored.
Can someone from Microsoft do a proper investigation and response about this?
This is a major data leak possibility (sign in to private and other corporate accounts), which is also not addressed through the "Tenant Restrictions" feature (which is about guest-account management and is not addressing actual other account sign-ins)