When you perform a cross-forest mailbox move request from Microsoft Exchange Server 2007 to Microsoft Exchange Server 2013, the mailbox is successfully moved. However, it finishes together with the CompletedWithWarnings status. The warning message resembles the following:
Warning: The following failure occurred after the active directory modification completed: couldn't find database "Exchange 2007 DB Name". Make sure you have it typed correctly.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.