When following the steps in the section "Create a new Hybrid user using web interface" in the Admin Guide, the resulting on-prem user is missing a primary SMTP address and the on-prem EAC displays an error message when viewing the properties of the on-prem object. The error message states that the user "has been corrupt or isn't compatible with Microsoft support requirements... There is no primary SMTP address".
The Exchange Online properties for the user are correct. Within the Active Roles Change History for the newly created user, the proxyaddresses attribute is getting altered with a value that has no SMTP: prefix.