When a computer is moved to the target domain, it will default to the new domain, however if you press escape or cancel the logon otherwise, it will default to the local computer name next time.
The behavior is by design of Microsoft OS architecture.
This is the same behavior as if a computer was moved to another domain with Microsoft native tools. Last logged in domain is stored in the registry based on the history of previously logged in name. Migration Manager is relying on native Microsoft APIs when moving computers between domains, but since computer was never in fact logged into the target domain, this name remains available for selection only the first time user logs in. If logon process is canceled for some reason, it will then default to the local machine name. Please complete the logon process immediately after moving computers to have it default to the target domain name at all times.