Username when connecting tenants
When 2 Azure tenants are connecting, if the external tenant users use their email to set up the account, it puts their email as the username in your tenant. Azure should update your tenant with their actual upn instead of email.
When they goto connect they need to use thier upn to log in but that information is not shown in your tenant and so you can not help them log in. They also can not reset the password because the email that shows as their username is not a valid account in your Azure AD Users.
This seems to occurs when UPNs do not match Email addresses in remote tenants.