Ok. I know there are some customers who are experiencing the issue still after applying CU8. I would strongly recommend opening a call with MSFT Support. Unfortunately I cannot share the case numbers I currently have on the issue, but the more people who open tickets on the issue, the more likely MSFT is to get data points and hopefully prevent the issue in the future.
Hi,
This issue might happen if original mailbox was in different language than the new mailbox. For example, original mailbox is EN-US and new database is EN-UK.
And please run the following command in Exchange powershell:
Get-MailboxFolderStatistics user@domain.com | Select Name,FolderSize
Are there two contact folders here as well?
I suggest to copy all old Contacts to new Contact and delete the old one.
Best Regards.
Circling back on this, It looks like an update was issued CU9 for this. I haven't applied it and tried a migration yet, but I've been told this update should prevent this issue from happening.