This is an expected behavior.
Only the Identity Source that is configured as the default domain within Single Sign-On (SSO) allows users to log in without specifying the full UPN (
user@domain) or the down-level logon name (
domain\user). By default, the Internal OS,
Local OS is configured as the default domain for SSO 5.5 and, therefore, logging in with administrator resolves to
administrator@<FQDNofSSOServer>. If multiple identity sources are configured for Single Sign-On 5.5, the users logging into the vSphere Web Client must specify their domain information either using a User Principal Name (UPN) or the down-level logon name as only a single default domain can be configured.