Email for Migrated Users "switching" back

We are going through a domain transformation which includes a email migration from a Exchange 2010 on premise setup into Office 365 using MMEX and a tenant to tenant migration using ODME.  The tenant to tenant migrations are going well but we are having big problems with the migration using MMEX.

We have gone through the initial steps of syncing the AD objects to out new domain, syncing the calendars and mailboxes into O365 but the issue we are having is after the mailboxes have been switched.  We are using version 8.14 of MMEX and after performing the mailbox switch everything seems to be fine.  Then we get reports that users are not receiving mail in Office 365.  After investigation we have noticed the the target address attribute in their AD accounts have been switched back to the legacy address setup to point to the on-premise mail  environment.  We are having to un-switch the users mailbox and then re-switch so that the targetaddress then rewrites to the Office 365 address.  So far we have migrated around 2000 users and not everyone is being affected by this.  We have had around about 50 users that have their mail switched back to their old mailboxes and we are getting more reports each day.

Any help would be much appreciated.