We have always had an issue with FIM not deleting old accounts when they still have active sync devices. We would run a script to remove those activesync devices, then it would complete successfully.
Now that we have migrated from Exchange on prem to Exchange Online, the script fails, but we still continue to have the issue deleting accounts.
If I manually run REMOVE-ACTIVESYNCDEVICES using the GUID on our on-prem Exchange server I get the error. "Couldn't find 'domain.com/OU/username' as a recipient."
How can I remove activesync devices from all users now that we are fully migrated to exchange online?