The issue is not ODM-related and can be seen in Microsoft Admin Center, clicking on Account, then on Mail tab. It's stuck in "We're preparing mailbox for the user" status for long time.
It is a Microsoft issue.
Please make sure that account's mailbox is fully provisioned by Microsoft prior to mailbox migration by ODM.
There is an article on how this issue can be addressed:
https://techieberry.com/we-are-preparing-a-mailbox-for-the-user/
