Move mailbox from 2010 to 2013, and then back to 2010 - cant then migrate to 2013 again!

Sorry for the long title, but I cant think of any other way of putting it. 

We are testing an Exchange 2010 - 2013 upgrade/migration and as part of the testing we migrated a mailbox from 2010 to 2013, and then moved it back to 2010 again - for testing purposes. Now, when we try and migrate it back from 2010 to 2013 we can select it, initiate the job, but within a couple of seconds the number of mailboxes to move drops to 0 (zero) and then the job completes, with no errors at all, but the mailbox is still on the 2010 Information Store.

Now, we dont plan on doing this but is this unique to use, or is this a bug/feature? Exchange 2010 is at SP3 with UR8MK2, and Exchange 2013 is with SP1/UR8.

Thanks

Phil

Phil