Service users got unexpectedly removed in the CSP Cloud after a successful Syndication-CSP migration.
The issue is recognized as APSA-18155 ([O365] During the synchronization the resource O365 User can be linked to the resource O365 domain which has the status aps:unprovisioning.)
The issue is permanently fixed in the Office 365 16.8.6 APS package. It is recomended to upgrade the Office 365 APS application to the latest version.
In order to work around this issue, it is necessary to stop the Office 365 Synchronization with Office 365 Portal periodic task for the CSP instance undergoing migration for the time of the procedure and start this task only after the Syndication subscriptions and domains are completely unprovisioned (all tasks are completed).