Hide User Information is a method of protecting user identities. With Hide User Information, an admin can configure the Vault to prevent unauthorized users from viewing user names and identifying details. However, user information is still visible with this feature enabled.
The unauthorized user's email address domain is the same as the target user.
This works as designed. With Hide User Information, users do not see the name or other identifying details of any Vault user that does not share the same email domain. Note that this is based on the user’s email address domain, not the Vault username domain.
Vault Help Documentation: About Hide User Information